Setup External Dependencies with Composer

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