Continuous Integration using TravisCI

This PR shouldn't be merged until after PR #105 

This implements the framework for automatic build and test execution using TravisCI.  You can see sample output from my fork [here](https://travis-ci.org/amcgee/MoMo-Firmware).  This also adds a current build status image to the README and automatically runs the CI sequence (specified in .travis.yml) whenever a pull request is opened.

Internally, Travis spins up a new VM, runs the provisioning script at /tools/vagrant/provision.sh to set up the development toolchain, and then runs all of the builds specified in .travis.yml by executing `momo build` in each module directory.

Currently, module firmware that requires the professional XC8 cannot be tested on this VM since XC8 is installed in Free mode, so pic12_executive and gsm_module are not built.  Eventually we should figure out a way to build these modules as well.