Modernise project configuration, add OS X unit test target, and convert Kiwi to XCTest

Ok! So this ended up being quite a few commits, but ultimately it all supports modernising the codebase onto XCTest and getting the tests green and solid, on both iOS and OS X.

Summary of the changes:

- Remove OCUnit support

- Simplify `Kiwi.podspec` by removing the XCTest/OCUnit split, and converting `KWMessagePattern.m` to ARC to remove the ARC/NonARC split

- Simplify build target structure and add OS X test bundle:

    ![capture 2014-06-23 at 9 16 23 pm](https://cloud.githubusercontent.com/assets/147686/3356855/df0313d0-fac7-11e3-968f-304c54a6ad75.PNG)

- Resolve remaining compiler issues