GSOC / Add of new message for OF1.3

Add : 
-  PortFeatures
 - PortConfig
 -MultipartReq (and its dependencies)
 
- add switch description reply (for multipart reply)
Yes I understand.
By removing this line (and in .ml, and the corresponding line in SDN_OpenFlow0x04.ml), it makes now an pattern-matching non exhaustive.