Adding changeSkinPartial, resetSkinPartial and colorizeSlots

Only required changes are to Spine.h and Spine.m; alterations to readme can be largely discarded as they'll overwrite the master readme (it's up to you as to whether to include the new documentation in the readme).

There may be a more efficient way of resetting the partially-changed skins utilizing the same dictionary used in the changeSkinPartial call; I tried comparing this against the default skin, but have had no joy so far.

If you need me to make any amendments before merging, let me know.