iOS-friendly static library

This pull request is based on https://github.com/thestk/stk/pull/8 and addresses Gary's [comments](https://github.com/thestk/stk/pull/8#issuecomment-37435171) on it.

 * There are no changes to any STK class. 
 * The iOS static library is now inside the `/iOS/` directory. It only consists of **STK.xcodeproj**, which appears as a single file to a OS X user, so I didn't find it necessary to place it inside a directory of its own. 
 * The iOS Demo project has been sent to the `/iOS/demo/` directory. The files it contains are the ones necessary to run it.