Added rake task for setting up database.yml

 - By default it will setup database.yml with postgresql as database.
 - Different adapter can be passed using ADAPTER ENV variable, and then
   database.yml for particular adapter will be setup.
 - Right now supports only MRI database adapters sqlite3, mysql2
   and postgresql.
Merge pull request #9 from bigbinary/add-database-yml-rake-task

 Added rake task for setting up database.yml