backwards compatibility for < iOS 5
Merge pull request #384 from graceful/ios7-warnings

Fix iOS 7 warnings for dismissModalViewControllerAnimated and presentModalViewController
Avoid warnings about implicit integer conversions that change the
signedness of an integer value (-Wsign-conversion).
Avoid warnings about @property declarations that are implicitly atomic
(-Wno-implicit-atomic-properties).
Merge pull request #342 from jweyrich/master

Fix warnings when compiling with some -W options.
Enable a couple of warnings for:
- Implicit integer conversions that change the signedness of an
  integer value (-Wsign-conversion);
- Implicit atomic @property declarations
  (-Wno-implicit-atomic-properties);
Merge pull request #388 from jweyrich/master

Enable a couple of warnings (related to #342)
build warnings