Adding regexCaptures to GCDWebServerRequest

I found it a little odd that there is regex matching for URLs but there didn't seem to be a simple way to get the captured variables. So I added a new property to GCDWebServerRequest called `regexCaptures` which will now contain those matches.

I tried to adhere to all the style and use of macros, but let me know if something is amiss. I haven't extensively tested all the various things that could go wrong, but from a little testing in my own server it seems to be working nicely. Here's an example usage that I used to verify it works:

    [webServer addHandlerForMethod:@"GET"
                         pathRegex:@"/1/test/(\\d+)/(.+)"
                      requestClass:[GCDWebServerRequest class]
                      processBlock:^GCDWebServerResponse *(GCDWebServerRequest *request) {
                          return [GCDWebServerDataResponse responseWithHTML:[NSString stringWithFormat:@"<html><body><p>%@ - %@</p></body></html>", request.regexCaptures[0], request.regexCaptures[1]]];
                      }];

I didn't update any pod spec details or other versions as that didn't seem appropriate, but I can if you'd like.
Use `-setValue:forKey:` instead so that you can clear an attribute when passing `nil`.