fixes #17 Add mongodb support

 * uses puppetlabs/mongodb
 * interface like mysql/postgresql
component::mongodb doesn't have a root_password parameter
Recommend setting mongodb::server::smallfiles to true

If there isn't at least 4GB of free storage, MongoDB will not start
without `smallfiles` being set in `mongodb.conf`.
Merge pull request #25 from Mayflower/mongo

Add mongodb support