Added Travis CI continuous integration

This patch provides continuous integration using the Travis CI build service. With setup of Travis, the new unit test cases can finally provide real value by providing unit test feedback on new commits.

To apply:
  - setup a volkszaehler account at https://travis-ci.org/
  - follow the instructions at http://about.travis-ci.org/docs/user/getting-started/ to enable the github service hooks
  - configure the desired services in `.travis.yml`:
    - comment databases not wanted in the `env` section
    - configure email preferences for failed builds in the `notifications` section
    - set branch for building to `master` in the `branches` section
- in `test/README.md` change the build status image link to match the new travis-ci user