Setup External Dependencies with Composer

In this PR I have...

- [ ] Removed Twig from the source
- [ ] Removed PHP-Router from the source
- [ ] Managed installation of Twig and PHP-Router via Composer
- [ ] Updated paths to point at the `/vendor` directory
- [ ] Updated Travis and readme with necessary composer steps

I have not (but would like to)
- [ ] Remove Twig_Cache_Extension from Source
- [ ] Manage installation of Twig_Cache_Extension from Composer
- [ ] Update paths, etc. to load Twig_Cache_Extension from `/vendor`