Work-in-progress Wireshark dissector

Reviewer: @andi-bigswitch

This is not fully functional, but I want to get it in the branch to allow other people to hack on it.

This is a LOXI backend that generates a Wireshark dissector for all versions of OpenFlow, including vendor extensions as defined in the LOXI input files. The generated dissector is a Lua file which can be installed into ~/.wireshark/plugins without needing to be compiled like a C dissector.

What works now:
 - Top-level messages are dispatched to their dissector functions based on version/type.
 - Most fields (including OF 1.0 of_match_t) are read and added to the dissection tree.
 - Child class tables keyed on discriminator value are populated for each virtual class.

TODO:
 - Replace virtual class dissectors with ones that read the discriminator field and dispatch to a child class dissector.
 - Show human-readable enum member names.
 - Support more field types, especially all the lists.
 - Everything else.