Refactor to use with memcached

In view to take advantage of the new features of the *memcached* library (note the last 'd'), I updated this bundle and added a small suite of tests.

I have included some improvements like:

* A method to get all the registered keys from the cache
* A method to flush the cache
* A method to remove cache entries with a regular expression
* A method to enable / disable compression
* Updated setPrefix method to take advantage of the new library
* A method to set multiple values in a single call using an array of pairs key => value
* A method to remove cache entries through an array of keys

I also added a simple way to configure the bundle following the standards of Symfony2.

Any questions or suggestions are welcome!

Thanks for this bundle!