Add travis ci support

This adds support for Travis CI.  

To turn on Travis:
- go to our travis page, and slide the switch for node-mapnik to 'on'
- go to the github setting page for node-mapnik (https://github.com/mapnik/node-mapnik/settings/hooks), click 'travis' and make sure user and token are set up 

Some notes:
- For both my local tests and those running on Travis, `npm test` would fail whenever a test had `require('mapnik')` rather than `require('../')`.
- I thought Travis would run `npm install` by default.  Travis didn't.
- I thought mocha would be installed globally by Travis, but it wasn't.


Fixes https://github.com/mapnik/node-mapnik/issues/173