Allow to use SLAVE_DATABASE_URL as connection URL for slave database.

It's hard to set up slave connection when DATABASE_URL environment URL is used instead of database.yml file.

I suggest to allow to use SLAVE_DATABASE_URL for slave connection.

In this case gem could be easier used within Foreman(https://github.com/ddollar/foreman) or DotEnv(https://github.com/bkeepers/dotenv)