Support enums in .msg files

[SchuellerSe]

Features:

• Supports enum definitions in .msg files, both on toplevel and scoped in structs/msgs.
• Make python2 explicitly the interpreter for the 'libucomm_wrap_msg' command. Ensures compatibility with systems using python3 by default.

Caveats / Mentions:

• enums on the toplevel of the .msg file are generated inside the Proto<> class.
  If enums without any scope are wanted, the 'custom' command can still be used.
• enums inside of structs in the .msg file need to be defined before the members of
  the struct. Could be fixed with fiddling a bit more with pyparsing if wanted.