Create the Rails-related database rake tasks

We should provide meaningful replacements for the default database related rake tasks (that is everything under the `db`-namespace). This ensures compatibility with exiting tools (i.e. Capistrano). At the same time this means we do not need a replacement for every tasks. In that case just implement a `noop`.

For further reference take a look at the [implementation of mongoid][1].


[1]: https://github.com/mongoid/mongoid/blob/master/lib/mongoid/railties/database.rake