add minimal tests. Fixes #114

Using test-kichen and BATS, this verifies that a run list of
["recipe[mongodb::10gen_repo]","recipe[mongodb::default]"] correctly
starts the mongodb service on Ubuntu 10.04, 12.04 and CentOS 5.9, 6.4.

Testing can be performed by running
```
bundle install
bundle exec berks install
bundle exec kitchen test --parallel
```