Cleanup token implementation and add token validation if available

There's a couple bugs with the current implementation of role_change_notify():

1. The token does not get properly listed under the 'tokens' array of hook_token_info().
2. Corrected to use the proper token standard of using a dash rather than underscore (user:role-changed instead of user:role_changed).
3. Since the user:role-changed token may be changed on other user objects like [node:author:role-changed] or [current-user:role-changed] we need to ensure that we provide the proper token replacement value, which is $tokens['role-changed'] in the $replacements array().
4. Token help UI has changed to use the token tree.
5. If token module is installed, why not use the token validation. :)