Adding encoding capability to outbound traffic

Here are first steps of adding encoding to Wisdom.

Done : 
* Add content encoding constants 
* Add content encoders in the content manager service (GZIP, DEFLATE, IDENTITY)
* Add content encoder retrieval in content engine (with encoding)
* Add Accepting_Encoding parsing (partially done) + encoding sorting based on order and qValue
* Add content encoding to result processing
* Add a configuration key to enable / disable encoding globally

ToDo : 
* Add annotations to enable / disable encoding for a route / controller
* Add "compress" support
* Add "non-standard" encoding ( http://en.wikipedia.org/wiki/HTTP_compression )
* Review Accepting_Encoding parsing as I'm not sure of some behaviors + "*;qValue" is not handled
* Unit tests

To discuss : 
* Default behavior : enabled / disabled
* Handle compressed inbound traffic
* Which "non-standard" encoding to support
* Should we allow a user to force a given encoding, discarding Accept_Encoding order

I also made a change to RenderableString, as the Charset was not uset to convert the string to byte. Need reviews.