[TITokenField] Added the ability to mark a token already used when showAlreadyTokenized = YES. Various refactors.

- Added (BOOL)markAlreadyTokenized and (UIImage *)markAlreadyTokenizedImage.
- When markAlreadyTokenized=YES and markAlreadyTokenizedImage = a valid UIImage and showAlreadyTokenized = YES
  the user will see the UIImage as a small icon next to tokens already taken in the resultsTable. Rather
  than adding the same token multiple times it will uncheck and delete a token if it was already selected.
- Added method - (TIToken *)tokenForRepresentedObject:(id)representedObject . This method grabs the token
  from the TITokenField for the representedObject. The code is essentially the same as the second enumerate objects block
  search process for the resultsForSearchString method. This chunk of code proved useful in other areas so I
  abstracted it out.
- Added KVO observation for the .tokens property of a TITokenField.
  I found myself commonly trying to reference changes in the tokens property in more places than just the delegate.
- Fixed shadow bug when forcePickSearchResults = YES. Shadow was dissapearing at incorrect times when.