Added httplib2 support.

Hi!

Congratulations for this great package!

I added httplib2 support because I need to test my libraries based on dRest, that is based on httplib2.

The tricky thing of this modification is that I had to change the parent class of `VCRHTTPResponse` from `object` to `HTTPResponse` because httplib2 is doing `issubclass` checkings against the response object. I don't know if this is correct but all the tests have passed.

Thank you!