Add db:reset and db:migrate:reset

This is the exactly the same as Rails' `rake db:reset` and `rake db:migrate:reset`.

`rake db:reset` runs `rake db:drop` and `rake db:setup`.
`rake db:migrate:reset` runs `rake db:drop`, `rake db:create`, and `rake db:migrate`.

I'm not entirely sure how to go about testing the Rake tasks so any help there would be greatly appreciated.