Rackspace adapter factory

Hi, this is my solution to implement Rackspace adapter factory.
It uses an new adapter based on the original Gaufrette Rackspace adapter that tries to be more configurable and aims to be lazy loaded.
I think the best solution would have been to modify the original Rackspace adapter in the main Gaufrette repository (library) but it would have create a backward compatibility problems. So this solution isn't probably the best but is fully compliant with the current library.

I've also updated the readme file to provide documentation on how to configure your services to adopt the new Rackspace adapter factory, but as you probably noticed, my english is really far from perfection, so feel free to correct it if you see some mistake.