Adds the ability to remove a symlinked sdk from the command line.

typical usage would be `fix-xcode remove 7` to make it so that Xcode only sees the iOS 6 SDK on the next restart. Running fix-xcode will restore the 7 SDK back. You can pass 7 or 7.0
Merge pull request #13 from markrickert/remove-sdks

Adds the ability to remove a symlinked sdk from the command line.