java_gen: renamed OFPortMap to OFPortBitMap

We could call it OFPortSet to emphasize that it is really a
set of ports. Map is definitely the wrong name though
as it evokes a Key->Value mapping.
java_gen: Add documentation on OFPortBitmap, unit test clarity fix

Limit the unit test use the to 127 (valid) ports for clarity.
Merge into master from pull request #99:
Rename and a few documentation/clarity fixes for OFPortBitMap (https://github.com/floodlight/loxigen/pull/99)