Storage management (mix task create/drop repo)

This is a PR for issue #97, the purpose is to add two new mix tasks: ecto.create Repo and ecto.drop Repo.

Here is a summary of what's going on:

- [x] new mix task ecto.create + unit test
- [x] PG implementation of storage_up + integration test
- [x] new mix task ecto.drop + unit test
- [x] PG implementation of storage_down + integration test 

It's ready for merge.

Thanks for the review!