Only include functions when executing phakefile

This is a suggested fix in order to be able to list `jaz303/phake` as a `require-dev` in your `composer.json`.

This minor patch changes it so that phake's global functions are no longer registered whenever the autoloader runs, but instead only when phake actually executes a `phakefile`. IMHO this is in line with what you'd expect when adding phake to one's project dependencies.

I consider this open for discussion, as I'm not *perfectly happy* with this solution either.

Fixes #33.