Added multipartFormRequest and refactored auth header

As you mentioned in my issue on AFNetworking (https://github.com/AFNetworking/AFNetworking/issues/815#issuecomment-13863968) the proper way to get a signed multipartFormRequest would be to override it, so I took a crack at doing so in AFOAuth1Client.

For tidy-ness I refactored the authorizationHeader stuff, since that function is only ever called from requestWithMethod, and I needed to do the same in multipartFormRequestWithMethod.

I tested by posting a photo to Tumblr and it seemed to work fine.

Let me know what you think!