add Composer support

This will allow installing Kostache with Composer. The Mustache.php
dependency can be autoloaded - just ensure you are including the
Composer autoloader file in your project.

This change should still be backwards compatible with using
Kostache as a Git submodule.
Merge branch 'feature/composer-support' of https://github.com/cmwelsh/KOstache
Removed mustache submodule. Renamed package name to modulargaming/KOstache.