Request Matching and Record Modes

Backwards incompatible release - Added support for record modes, and changed the default recording behavior to the "once" record mode. Please see the documentation on record modes for more. Added support for custom request matching, and changed the default request matching behavior to match only on the URL and method. Also, improved the httplib mocking to add support for the HTTPConnection.send() method. This means that requests won't actually be sent until the response is read, since I need to record the entire request in order to match up the appropriate response. I don't think this should cause any issues unless you are sending requests without ever loading the response (which none of the standard httplib wrappers do, as far as I know. Thanks to @fatuhoku for some of the ideas and the motivation behind this release.