Add yaml file for controlling the continuous integration service.

 * This is currently hooked to my personal repo, for testing purposes, before
   we decide whether or not to hook it to the common organization repo.
 * The continuous integration is pretty basic so far: it checks with python
   versions 2.6, 2.7, and 3.3, that we are able to install everything in the
   requirements.txt file, that "$ ./inputs/make" works, and then that the
   following test:
   "$ bin/ooniprobe nettests/blocking/dnsconsistency.py \
        -f inputs/ooni-inputs/processed/alexa-top-1k.txt"
   completes successfully.
 * It uses Travis-CI, see http://about.travis-ci.org/docs/user/getting-started/
Add --use-mirrors to the TravisCI install command, as is recommended.
Merge branch 'feature/continuous-integration' into develop