Add support for configurable record modes

This support will let you select one of four different behaviors
for VCR's cassettes.  Closes #23

tests/integration/test_record_mode.py

@@ -0,0 +1,86 @@
+import os
+import urllib2
+import pytest
+import vcr
+
+
+def test_once_record_mode(tmpdir):
+    testfile = str(tmpdir.join('recordmode.yml'))
+    with vcr.use_cassette(testfile, record_mode="once"):
+        # cassette file doesn't exist, so create.
+        response = urllib2.urlopen('http://httpbin.org/').read()
+
+    with vcr.use_cassette(testfile, record_mode="once") as cass:
+        # make the same request again
+        response = urllib2.urlopen('http://httpbin.org/').read()
+
+        # the first time, it's played from the cassette.
+        # but, try to access something else from the same cassette, and an
+        # exception is raised.
+        with pytest.raises(Exception):
+            response = urllib2.urlopen('http://httpbin.org/get').read()
+
+
+def test_new_episodes_record_mode(tmpdir):
+    testfile = str(tmpdir.join('recordmode.yml'))
+
+    with vcr.use_cassette(testfile, record_mode="new_episodes"):
+        # cassette file doesn't exist, so create.
+        response = urllib2.urlopen('http://httpbin.org/').read()
+
+    with vcr.use_cassette(testfile, record_mode="new_episodes") as cass:
+        # make the same request again
+        response = urllib2.urlopen('http://httpbin.org/').read()
+
+        # in the "new_episodes" record mode, we can add more requests to
+        # a cassette without repurcussions.
+        response = urllib2.urlopen('http://httpbin.org/get').read()
+
+        # the first interaction was not re-recorded, but the second was added
+        assert cass.play_count == 1
+
+
+def test_all_record_mode(tmpdir):
+    testfile = str(tmpdir.join('recordmode.yml'))
+
+    with vcr.use_cassette(testfile, record_mode="all"):
+        # cassette file doesn't exist, so create.
+        response = urllib2.urlopen('http://httpbin.org/').read()
+
+    with vcr.use_cassette(testfile, record_mode="all") as cass:
+        # make the same request again
+        response = urllib2.urlopen('http://httpbin.org/').read()
+
+        # in the "all" record mode, we can add more requests to
+        # a cassette without repurcussions.
+        response = urllib2.urlopen('http://httpbin.org/get').read()
+
+        # The cassette was never actually played, even though it existed.
+        # that's because, in "all" mode, the requests all go directly to
+        # the source and bypass the cassette.
+        assert cass.play_count == 0
+
+
+def test_none_record_mode(tmpdir):
+    # Cassette file doesn't exist, yet we are trying to make a request.
+    # raise hell.
+    testfile = str(tmpdir.join('recordmode.yml'))
+    with vcr.use_cassette(testfile, record_mode="none"):
+        with pytest.raises(Exception):
+            response = urllib2.urlopen('http://httpbin.org/').read()
+
+
+def test_none_record_mode_with_existing_cassette(tmpdir):
+    # create a cassette file
+    testfile = str(tmpdir.join('recordmode.yml'))
+
+    with vcr.use_cassette(testfile, record_mode="all"):
+        response = urllib2.urlopen('http://httpbin.org/').read()
+
+    # play from cassette file
+    with vcr.use_cassette(testfile, record_mode="none") as cass:
+        response = urllib2.urlopen('http://httpbin.org/').read()
+        assert cass.play_count == 1
+        # but if I try to hit the net, raise an exception.
+        with pytest.raises(Exception):
+            response = urllib2.urlopen('http://httpbin.org/get').read()