Modernizes project and sample for iOS6 / iOS7.

Removes old, unnecessary kludges required for targeting
Twitter.framework and Social.framework which were required for iOS5
support. Also updates TWSignedRequest to make synchronous call the way
that one should with iOS6+ support.

tl;dr no more iOS5 makes everything better
Removes assert that was failing on GET requests.

Resolves #11.
At some point I guess I should heed my own warnings.