Add flake8 tool.

Its very similar to pep8, there is probably some room for
fixing the code duplication.
Update README for flake8
Add first draft of phpcs checker.

It only supports the standard option right now.
More to come.
Add a few other options I've used in the past

I've used these options in the past. I'll assume other people
will use them too.
Update README for phpcs.
Try and install various tools into travis.