added (request-target) support to be able to use the library for the ietf draft

I've added support for (request-target) so that this library can also support this spec: http://tools.ietf.org/html/draft-cavage-http-signatures-03

There's a bunch of http-signature libs around and their implementations all float around a bit between that draft and the joyent spec, if you could at least support that spec a bit more too then that would be nice!