Adding option to remove all tokens at once.

In using this library, I found it useful to add a function to remove all tokens at once. I had an instance of `RMSTokenView` in a collection view cell, and in order to provide clean reuse (in `-[UICollectionViewCell prepareForReuse]`), I wanted to a way to clear out the token views in use.

Let me know if you'd like anything else with this PR request; I'm happy to accomodate.