Allow contrib fixers + example Fixer.

Due to contrib namespace idea mentioned in #381 and touched again in #397.
Lot of work was already done, here putting it all together and add some more.

I have added Contrib subnamespace in Fixer namespace (Symfony\CS\Fixer\Contrib), allow Fixer to register them and add CONTRIB_LEVEL (which not include PSR1, PSR2, all).
Finally added a Fixer made by sstok about 3 weeks ago (#381) and updated by me to use current form of Token/Tokens.

What do you guys think?