Add basic Travis CI configuration

I added a basic Travis CI configuration to make the build status more
transparent. I added a small logo to the bottom of the README with the
status and a link to the build. @evancz after merging this you'd have to
go to https://travis-ci.org and enable the build for this project.

I hope that we can add something like this so that we can quickly take
care of failures and not merge branches that fail the test suite (NB -
the test suite is currently failing locally for me and on Travis right
now).