Adding a composer packagist file

[mcloide]

Having this repo under packagist (https://packagist.org) will make the cloning and installation of it faster and simpler if you are already using composer. With this file on the repo all that will be necessary is to add the new package at packagist and from there packagist will handle any update.

[marcoceppi]

This breaks backwards compatibility with all the .icon-large definitions. Adding a .glyphicons class is fine but please don't break the existing .icon definition