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, and used __IPHONE_OS_VERSION_MIN_REQUIRED to figure out when to call the older version
- UITextAlignment vs NSTextAlignment. I've used the __IPHONE_OS_VERSION_MIN_REQUIRED macro to figure out which one to use.
- dispatch_get_current_queue were for asserts, which get removed on release builds, so I simply added the relevant #pragmas to them too.

Let me know if I should change anything to comply with project coding standards or whatever.
I did sign the CLA