Add task to show migration status

This is similar to rails task "db:migrate:status".
The output would be like:

 Status   Migration ID    Migration Name
--------------------------------------------------
   up     20130101000000  Create sites
   up     20130102000000  Create bookmarks
  down    20130201000000  Add score to sites
Merge pull request #17 from eitoball/add_task_to_show_migration_status

Add task to show migration status

Patch by Eito Katagiri. Thanks!!!