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/__init__.py

@@ -1,5 +1,17 @@
+import logging
 from .config import VCR
 
+# Set default logging handler to avoid "No handler found" warnings.
+import logging
+try:  # Python 2.7+
+    from logging import NullHandler
+except ImportError:
+    class NullHandler(logging.Handler):
+        def emit(self, record):
+            pass
+
+logging.getLogger(__name__).addHandler(NullHandler())
+
 default_vcr = VCR()