Create a local copy of 'type' in the parser

When reading a protobuf message with only one field, I get the following error (Lua 5.2):

  pb/proto/parser.lua:82: attempt to call global 'type' (a nil value)

Making a local copy of function type in the parser fixes the problem.
Merge pull request #13 from janakj/patch-1

Create a local copy of 'type' in the parser