Add .travis.yml

Since travis doesn't natively support multiple languages (see:
http://stackoverflow.com/questions/18456611/is-it-possible-to-set-up-tra
vis-to-run-tests-for-several-languages), we ask for python 2.7 and we
use whatever version of node is installed on the base image.
Merge pull request #12 from mattmo/travis

Add .travis.yml