Add travis-ci build script.
(cherry picked from commit cfe357a546669c3ca70ee1f65dbc5034c2be4760)
Update .travis.yml to put the install commands under the install section.

 * Separate commands for testing pip and setuptools installations.
 * TODO I believe there is a way to tell TravisCI to try both, we should check
   the docs.

travis-lint results:
(ooni)∃!isisⒶwintermute:~/code/torproject/ooni-probe ∴ travis-lint
Hooray, /home/isis/code/torproject/ooni-probe/.travis.yml seems to be solid!
(ooni)∃!isisⒶwintermute:~/code/torproject/ooni-probe ∴ date
Mon Mar 11 21:00:27 UTC 2013