GC support and test targets

Here is my take on adding GC support:

1. Added GCC_ENABLE_OBJC_GC supported to the main library.
2. Added a GC only test target (built with GC required) and configured it to run after the existing MacOS test.
3. Used preprocessor to call either JSONKit or NSJSONSerialization as appropriate.
4. Removed comments from the JSON test suite data as they cannot be loaded by NSJSONSerialization. Loosing the comments is lamentable and could perhaps be avoided by running the JSON strings through a regex prior to de-serialising.

All tests pass including the iOS ones. In app integration should reveal if there are any issues lurking.

I have not updated the built binary that is included in the repo nor have I commited a version tag.