Add support for regex URL matching

Hi guys,

Thanks for the library!

I've added support for regex URL matching, as requested in dropbox/responses#20.

The only caveat at the moment is that this change is not be fully backwards compatible for people relying on exact string comparison. One of the existing tests failed due to the fact that matching against a regular expression is more "generic" than matching against a string. The failure example is matching `http://example.com/foo` against a regular expression, compiled with the string `http://example.com`. The fix was to change the regular expression to `http://example.com$`, as the diff shows.

I needed this for my own use case, hence the PR. Let me know if this is acceptable or not. If not, maybe we can come up with a better solution.

Cheers.