Handle empty project yml

[eschwartz]

This pull request includes:

• Add integration tests using mocha.
  The integration tests run against a set of tmstyle/tmsource project file fixtures,
  and run all the way through the process of tile image generation.
  You can run the tests via npm test
  Motivation: I am interested in diving a little deeper into this project. However, there
  is enough functionality I don't understand, that it makes it a little scary to touch the code.
  Having some integration tests makes this less scary.
  Having more coverage would be great (I'm only testing a thin slice here).
• Handle the error when an empty project.yml file is passed to tilelive-tmstyle.
  Previously, this resulted in an uncaught exception.
  Motivation: I'm using tilelive-tmstyle to power a tile server. We have been experiencing
  API timeouts on a regular basis, which have been really hard to track down.
  My suspicious is that uncaught exceptions within tiielive-tmstyle / tilelive-tmsource
  may be one of the causes of these timeouts.
  Better exception handling will allow me to log and track down the root cause of these errors.