Add showDatabase method

[wzrdtales]

Refers to db-migrate/node-db-migrate#339

showDatabase should reply with an array of database name strings.

--- Want to back this issue? **[Post a bounty on it!](https://www.bountysource.com/issues/29956554-add-showdatabase-method?utm_campaign=plugin&utm_content=tracker%2F12292998&utm_medium=issues&utm_source=github)** We accept bounties via [Bountysource](https://www.bountysource.com/?utm_campaign=plugin&utm_content=tracker%2F12292998&utm_medium=issues&utm_source=github).