First steps to add encoding capability to outbound traffic

**Need Review**
**Merge Candidate**

Here are first steps of adding encoding to Wisdom as stated in #10 

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 (lots of TODO in code)
* Add content encoding to result processing
* Add a configuration key to enable / disable encoding globally
* Add annotations to enable / disable encoding for a route / controller
* Add size limit to @AllowEncoding and global configuration (code is still rough)
* Review Accepting_Encoding parsing as I'm not sure of some behaviors + "*;qValue" is not handled
* Don't work on /assets/xxx routes, they have a length of -1 => Not RenderableFile but RenderableStream. Added a configuration key to allow encoding of ALL stream. Still disable with the @DenyEncoding annotation.
* Documentation for annotations, default behaviour
* Filter on mime types
* Stream are not encoded (no size no mime type). RenderableUrl are encoded if encoding.url is set and mime type not in exclude list
* Filter on already encoded files (Content_Encoding set and different from identitity)
* Unit tests (done for encoders)
* Documentation
* Encoding is now asynchrone

ToDo : 
* Check encoding behavior with cache (Compressed each time or the result is cached ?)

Future Work : 
* Add "compress" support
* Add "non-standard" encoding ( http://en.wikipedia.org/wiki/HTTP_compression )
* Which "non-standard" encodings to support
* Handle compressed inbound traffic

To discuss : 
* ——Default behavior : enabled / disabled—— => *enabled*
* --Configuration key name. Currently : global.encoding, encoding.max.size, encoding.min.size-- **encoding.global, encoding.max.size, encoding.min.size, encoding.url**
* --Should we allow a user to force a given encoding, discarding Accept_Encoding order-- **no**
* --Filter on file extensions ? (http://www.fileinfo.com/filetypes/compressed)-- **on mime type**
* --Default minimum / max size : http://webmasters.stackexchange.com/questions/31750/what-is-recommended-minimum-object-size-for-gzip-performance-benefits-- Ok 10ko - 10Mo

I also made a change to RenderableString, as the Charset was not used to convert the string to bytes. Need reviews. (I think it is linked to #42)