Changed type of port list in features reply to be OFPortDesc
Added STP state field to OFPortState
Support for overriding toString method for enum values

- added toString overrides for OFPortFeatures, OFPortState and OFPortConfig
Removed toString overrides for port-related classes in Java
Merge into master from pull request #64:
Changed type of port list in features reply to be OFPortDesc (https://github.com/floodlight/loxigen/pull/64)