ARC, latest AFNetworking, updated example projects and bug fixes

I didn't want to make a pull request about this but since I got lots of mails asking to (don't know why they can't just use my repository :/ ) so here it is. The reason I didn't want to push this is because I don't like some minor design choices and I plan on rewriting part of the core to make it more similar to AFOAuth2Client and to add some kind of token persistency, but since I'm pretty busy that will wait.

In the meantime this will suffice, I fixed a memory leak, made it arc compatible, updated the example projects and also fixed a bug that prevented the authentication with most OAuth provider (Twitter is too permissive, even broken code works with their API :/ )
It also solves all issues reported.