add WhitespacyCommentTransformer

Separate trailing whitespacy chars after comment into separate, following T_WHITESPACE token.

Will help to fix #774 [see also](https://github.com/FriendsOfPHP/PHP-CS-Fixer/issues/774#issuecomment-64912296).

Simplify few issues when changing FixerInterface (see #902)
why not, done