Allow regular expression URI matching from a JSON recording

As described in dstnbrkr/VCRURLConnection/issues/23, my server's API uses time-based hashes in the URI, which breaks the recoring-lookup by VCR.

Because the URI structure is always the same, with merely the hash changing, I wanted to propose allowing regular expression matches in the recorded JSON files to get VCR to work with expected changes in the URI.

I've added three test methods to cover the added behavior, without breaking the existing one.

Is this something you'd consider merging, @dstnbrkr?
Should we use a field other than 'uri' for regex matchers? If we can do that, then we keep the regexes out of the responseDictionary (good since they'll never be a valid key) and we only enumerate the regexes.