Add Logging

This helps to figure out which matcher has decided your two cassettes
differ, and figure out when your cassettes have hit the network.

Closes #34

vcr/matchers.py

@@ -1,3 +1,6 @@
+import logging
+log = logging.getLogger(__name__)
+
 def method(r1, r2):
     return r1.method == r2.method
 
@@ -22,5 +25,12 @@ def headers(r1, r2):
     return r1.headers == r2.headers
 
 
+def _log_matches(matches):
+    differences = [m for m in matches if not m[0]]
+    if differences:
+        log.debug('Requests differ according to the following matchers: {0}'.format(differences))
+
 def requests_match(r1, r2, matchers):
-    return all(m(r1, r2) for m in matchers)
+    matches = [(m(r1, r2), m) for m in matchers]
+    _log_matches(matches)
+    return all([m[0] for m in matches])