Avoid overwriting rails db:* rake tasks

Related to #312.

I started with the smallest step possible – removing the code that overrides the default `rake db:*` tasks. After this change the `db` namespace only affects the database specified in `database.yml`, while the `riak` namespace affects the riak clusters.

Do you think there is value in introducing another set of tasks, that would work on both databases at the same time (basically bringing the old behavior back under a different namespace)?