Use global variables for generated *_pb.lua
Use global variables for generated *_pb.lua

- Workaround the internal cap on local variable count enforced in the lua VM
- Fixes #13

Change-Id: I67fc84c2de6d978b7f57aed62c04e75a11d079ba

zh423328/protoc-gen-lua,zh423328/protoc-gen-lua