VTG encoding

I needed support for VTG and MWV encoding, so I added it. Includes mocha tests for encoding and decoding these messages.

I started breaking the code to smaller pieces by moving MWV and VTG stuff to a separate codec files. Also first steps for managing NMEA sentence ids and json message type ids are there.

For encoding utilities I included https://github.com/nherment/node-nmea/blob/master/lib/Helper.js.

The code is not very general in terms of supporting every bit and piece of VTG fields, but works for me in piping data to iRegatta Pro on iPhone/iPad.