Add auto-complete source and setup function

This makes using the package easier for auto-complete users, as discussed in issue #11. My apologies for taking so long to do this.

The `eval-after-load` definition of the ac-source was based on [tern's auto-complete source](https://github.com/marijnh/tern/blob/master/emacs/tern-auto-complete.el), since it was the first thing I found in my ELPA packages with an ac-source. If there's another approach you were thinking of, just let me know.

If you're happy with the way this works, I'll update the docs. Users should be able to just call

    (payas/ac-setup)

in their php-mode hook.