added support in the Verifier for the 'Authorization' header

I'm not very good at reading specs so I might be wrong, but afaik it should be possible to use the 'Authorization' header without the 'Signature' header...  
At least some libraries in other languages (python for example) don't send the 'Signature' header along, only the 'Authorization' so it would be nice if the Verifier would be able to deal with that!

Even if it's not completely acording to spec (I'm not sure, I get dizzy whenever I open that http://tools.ietf.org/html/draft-cavage-http-signatures-03#section-2.1.3 page) this doesn't break anything and it does add support for the slightly different implentations!