Substantial refactoring

This refactoring includes some PEP-8 compliance changes, as well as some more
complete testing to ensure that we're in fact serving everything out of
cassettes when we thing we are.

Incidentally, it also includes fixes for #3 and #4

vcr/__init__.py

@@ -1 +1,6 @@
-from .patch import use_cassette
+# Import Cassette to make it available at the top level
+from .cassette import Cassette
+
+# Also, make a 'load' function available
+def use_cassette(path):
+    return Cassette.load(path)