Update the Touchstone category

Fix some logic bugs, improve some comments and add @dynamic accessors.
Update example to use the category.
Remove the Touchstone standalone class.

This class introduced more problems than it helped to solve. First, you
had to use this class singleton _everywhere_ you had previously used
NSUserDefaults. This meant lots of code changes to use Touchstone. By
using a category that swizzles NSUserDefaults, we can get the same
behavior, without resorting to massive code refactoring. Just include
the NSUserDefaults+Touchstone.h header when you setup your
NSUserDefaults registration defaults. So probably in one place.
Improved and updated README
Simplify the non-volatile registration code.