loxi_frontend: move classes only used by c to c_gen

 - flags
 - identifiers
 - match
 - translation
 - type_maps

were only used by the c backend. I'm moving them to c_gen to
unclutter the loxi_front_end space (and updating references
in the c backend)
moved of_g to c_gen.of_g_legacy, introduced loxi_globals

as of_g includes loads of legacy data (and a representation that
is increasingly tailored to the c backend only), I've moved it to
c_gen.of_g. In its place, I have created loxi_globals, which gives
access to the list of versions and the IR.

Also loxi reorg: move command line processing to cmdline.py
loxi_ir: Moved to its own package

to group ir and related functionality. Also introduce a dedicated
frontend_ir model, and adds a unit test
loxi_utils: use new ir, split out template_utils
backends: use template_utils
py_gen: adapt to of_g removal / new ir model
java_gen: adapt to of_g removal, new ir
java_gen: add logback, logback config
group_mod: add subclass structure

the subclass structure is hidden from the c_gen of_g
openflow_input: fix superclass for of_experimenter_error_msg
Makefile: use nosetests for check, add ctags target
Merge branch 'master' of github.com:floodlight/loxigen into group_mod

Conflicts:
	java_gen/java_model.py
	loxi_front_end/frontend.py
	loxi_front_end/frontend_ir.py
	loxigen.py
	py_gen/templates/message.py
	utest/test_frontend.py
loxi_ir: two fixes
wireshark_gen: adapt to new of_g
loxi_utils: remove duplicated / unused methods
Infrastruture: fix typos
loxi_ir: minor fixes to address reviewer comments
Merge branch 'master' of github.com:floodlight/loxigen into group_mod
add setup.cfg for nosetests to find its unittests
extended README.md