Autoload all user-facing functions, see #469

Diffs and title should explain what I'm doing.

I'm guessing that the random autoloading of a few functions, but nothing else is not on purpose, and that this would be an improvement to projectile. In particular, this would allow me to load `projectile` lazily, speeding up emacs. It would also help people who do not `(require 'projectile)` to use projectile functions anyway.