Add mix tasks to setup database without Ecto

[adolfosilva]

This PR tries to simplify the database setup needed by Rihanna by providing an easy way to run the necessary migrations instead of asking users who are not using Ecto to do it manually.

Introduces the following changes:

• four new mix tasks mimicking the Ecto database setup tasks;
• an alias rihanna.setup (mix do rihanna.create, rihanna.migrate);
• updates CONTRIBUTING.md
• updates documentation

$ mix help --search rihanna
mix rihanna.create  # Create the Rihanna database
mix rihanna.drop    # Drop the Rihanna database
mix rihanna.migrate # Runs the necessary Rihanna migrations
mix rihanna.setup   # Alias defined in mix.exs
mix rihanna.upgrade # Upgrade an existing Rihanna database

[samsondav]

Hmm... not sure how I feel about this approach.

@sadir @lpil any strong feelings?

[lpil]

I think that any projects not using ecto migrations probably has another way of managing database schema changes so I'm not sure how useful these mix tasks would be.