Remove git submodules and update composer.json to install submodules

See http://fuelphp.com/forums/discussion/13015/fuelphp-1-x-composer-setup

`composer.json` in this PR is for 1.7.2<del>, and needs tags "v1.7.2" of all sub repositories</del>.

To test this, change all <del>`"reference": "v1.7.2"`</del>`"reference": "1.7/master"` to `"reference": "1.8/develop"`