New Xcode setup guide

This is a new Xcode setup guide, updated for Xcode 6 and includes super necessary modern technologies like gifs.

It includes a bit about making sense of errors, using breakpoints effectively and a pretty elaborate bit on adding addons. I erred on the side of not mentioning OFPlugin, since I figured that might be a little presumptuous? I basically just codified what its internal logic is in english for the addons section :sweat_smile:

I tried to be reasonably intelligent about not dating it too hard with regards to specifics about Xcode itself and the project generator.

Closes #197