Fix clang warnings when target iOS6+

Hi,

This pull request fixes all clang warning when targetting iOS 6+.
They fell in two categories:
- UIViewController present methods. I've added the relevant respondsToSelector checks, plus relevant #pragma statements
- UITextAlignment vs NSTextAlignment. To keep the project building on ios 5 targets, I've simply added #pragmas to these.

Let me know if I should change anything to comply with project coding standards or whatever.