Add pliny-db-* commands

Adds the following executables to Pliny based on the Rake tasks and miscellaneous executables that we had before in the template:

* `pliny-db-create` -- Creates dev and test databases.
* `pliny-db-drop` -- Drops dev and test databases.
* `pliny-db-dump` -- Dumps database schema to db/schema.sql.
* `pliny-db-load` -- Loads dev and test database schema from db/schema.sql.
* `pliny-db-merge` -- Resets databases, runs migrations, dumps schema, * and removes migrations.
* `pliny-db-rollback` -- Rolls back a migration.

/cc @mfine @pedro Does this look like the right way forward to you?