Enable 'multipart/form-data'

Enable 'multipart/form-data' on handler.
Move the getRouteParam method to Route to be able to override the default deserialization. It allows to parse the different parts as you can see in the test. 

Resources : 
  * http://dev.eclipse.org/mhonarc/lists/jetty-users/msg03294.html
  * http://www.codejava.net/java-se/networking/upload-files-by-sending-multipart-request-programmatically