Add support for custom request matchers

This commit not only changes the default method of matching requests
(just match on method and URI instead of the entire request + headers)
but also allows the user to add custom matchers.

vcr/matchers.py

@@ -0,0 +1,26 @@
+def method(r1, r2):
+    return r1.method == r2.method
+
+
+def url(r1, r2):
+    return r1.url == r2.url
+
+
+def host(r1, r2):
+    return r1.host == r2.host
+
+
+def path(r1, r2):
+    return r1.path == r2.path
+
+
+def body(r1, r2):
+    return r1.body == r2.body
+
+
+def headers(r1, r2):
+    return r1.headers == r2.headers
+
+
+def requests_match(r1, r2, matchers):
+    return all(m(r1, r2) for m in matchers)