Initial PyLoxi 1.3 work

Reviewer: @nbastin
CC @jonstout

These changes get the PyLoxi of13 module to parse and pass basic tests. OXM is added as its own sub-module, and core LoxiGen gets a minor change to support HELLO elements.

Some remaining work items:
 - Populate type fields for many classes (adding them to `get_type_values` in codegen.py).
 - Add unpack functions for several list types (`of_list_bucket_t`, `of_list_group_desc_stats_entry_t`, `of_list_group_stats_entry_t`, `of_list_meter_stats_t`, `of_list_table_features_t`)
 - Ensure all classes can pass trivial pack/unpack/show operations (see TestAllOF13 in py_gen/tests/of13.py).
 - Add pack/unpack tests for every message to of13.py.