Add Support for Requests Library

Added support and tests for the Requests library.  At least basic
support seems to be working.  The tests should run fine if you
don't have requests installed; it just won't verify the requests-
specific tests.

vcr/stubs.py

@@ -12,15 +12,28 @@ class VCRHTTPResponse(object):
         self.recorded_response = recorded_response
         self.reason = recorded_response['status']['message']
         self.status = recorded_response['status']['code']
+        self.version = None
         self._content = StringIO(self.recorded_response['body']['string'])
 
         self.msg = HTTPMessage(StringIO(''))
         for k, v in self.recorded_response['headers'].iteritems():
             self.msg.addheader(k, v)
 
+        self.length = self.msg.getheader('content-length') or None
+
     def read(self, chunked=False):
+        # Note: I'm pretty much ignoring any chunking stuff because
+        # I don't really understand what it is or how it works.
         return self._content.read()
 
+    def isclosed(self):
+        # Urllib3 seems to call this because it actually uses
+        # the weird chunking support in httplib
+        return True
+
+    def getheaders(self):
+        return self.recorded_response['headers'].iteritems()
+
 
 class VCRConnectionMixin:
     def _load_old_response(self):