adding XcodeBuildSettings to hold constants for all settings.

Would prefer this over having the same strings duplicated all over the
code base.
Refactored getAvailableSDKAndAliases to use NSScanner

[-] Changed default handler to adapt to new way in which SDK information is being called thereby not breaking the existing tests
[-] Cleaned up code per fpotter's comments
[-] Revised testGetAvailableSDKsAndAliases so that FakeTask actually gets called
[-] Per fpotter's request, created new GetAvailableSDKsInfo function that returns all information found within xcodebuild -sdk -version and created new test case
[-] Refactoed GetAvailableSDKsAndAliases to use new function
[-] Cleaned up code and added comments
[-] When rebasing, deleted a line by accident
[-] Minor edits and formatting changes per fpotter's 2013-11-25 review
[-] Refactored the parsing to use regex to find the SDK version
tiny cleanup in AddSDKToDictionary()

I think this is a little more straight forward.