Objective-C Sample: OAuth lib rewritten to use ARC

The current lib we use is throwing a bunch of warnings related to the fact that it was written before ARC came along. Besides, it included a lot of files that we weren't using for the OAuth itself.

What I did:

- Kept the crypto files intact (hmac, base64, sha1)
- Kept two of the 3 categories used by the OAuth lib (only one method was used from the third one, I copied it into YPMutableURLRequest)

- The classes we used were OAToken, OAConsumer, OARequestProvider, OAHMAC_SHA1SignatureProvider, OASignatureProvider, OAMutableURLRequest. Deleted all the other ones since they weren't used by any of the code we had.

- Created new YP classes for all of the above mentioned classes. Kept only the methods that we used from them and rewrote them to use ARC. (Also made some of them a bit clearer in the process)

Tested the code and it works as it did before, and all the headers are set the way they were set before. 
The only warnings left are these attached in the screenshot here: ![Warnings](https://cloud.githubusercontent.com/assets/3008124/4686148/34167dd2-5642-11e4-86bc-0c39e479bfa4.png)