VPPAPIGEN: vppapigen replacement in Python PLY.

This is a version of the VPP API generator in Python PLY. It supports
the existing language, and has a plugin architecture for generators.
Currently C and JSON are supported.

Changes:
 - vl_api_version to option version = "major.minor.patch"
 - enum support
 - Added error checking and reporting
 - import support (removed the C pre-processor)
 - services (tying request/reply together)

Version:
 option version = "1.0.0";

Enum:
 enum colours {
   RED,
   BLUE = 50,
 };
 define foo {
  vl_api_colours_t colours;
 };

Services:
 service {
  rpc foo returns foo_reply;
  rpc foo_dump returns stream foo_details;
  rpc want_stats returns want_stats_reply
      events ip4_counters, ip6_counters;
 };

Future planned features:
 - unions
 - bool, text
 - array support (including length)
 - proto3 output plugin
 - Refactor C/C++ generator as a plugin
 - Refactor Java generator as a plugin

Change-Id: Ifa289966c790e1b1a8e2938a91e69331e3a58bdf
Signed-off-by: Ole Troan <ot@cisco.com>