Fully integrate composer

This PR makes it so Kohana modules must be installed using composer instead of using git submodules. 

Modules with the composer type of "kohana-module" are forced to be installed into the standard vendor directory.
@lenton Personally, I prefer to have a `VENPATH` global constant at this point. I know you're against the idea, and I will not insist. Because the way composer works, it just feels less dangerous than the other path, also it feels more *consistant*, because we have the other paths already.