Hierarchical error messages plus some more tweaks

- hierarchical error messages for different error types
- renamed ofp_stats_types to ofp_stats_type
- renamed OFP_VENDOR to OFP_EXPERIMENTER in OF 1.0 input file
- use ofp_capabilities enum in of_features_reply
- use boolean type for l2_table_enable field in of_bsn_set_l2_table_(request|reply)
- return Set<OFActionType> for actions field in of_features_reply
- added DatapathId class that's returned for datapath_id field of of_features_reply
- removed unused read/write routines from OFBufferId
- added getOFVersioon method to factory classes/interfaces
- renamed OFPT_MULTIPART_(REQUEST|REPLY) to OFPT_STATS_(REQUEST|REPLY) in OF 1.3 input file
- renamed ofp_multipart_types to ofp_stats_type in OF 1.3 input file
- tweaked python backend to work with consistent use of of_stats_xyz vs. of_multipart_xyz in 1.3
Merge branch 'master' of github.com:floodlight/loxigen
Fix issues with C backend with new error message hierarchy
Test two different error types in error message unit tests

- Added coverage for C and Java backends to the error msg tests
Fixed bug in C versions of error message unit tests
Merge branch 'master' of github.com:floodlight/loxigen

Conflicts:
	java_gen/java_type.py
Fixed issues from code review
Merge branch 'master' of github.com:floodlight/loxigen

Conflicts:
	java_gen/java_type.py
Merge into master from pull request #80:
Error message type hierarchy plus renamed some things for consistency across OF versions (https://github.com/floodlight/loxigen/pull/80)