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 scrap that, it's because the [unittest bootstrap](https://github.com/kohana/unittest/blob/3.3/master/bootstrap.php) defines it's own set of path constants and so is still pointing to /system. I think we need to fix that separately (I can take a look as part of koharness), we'll just have to accept that this commit breaks the build till that's resolved.