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.
 - Connection pool size can also be configured using POOL_SIZE ENV
   variable.
 - Right now supports only MRI database adapters sqlite3, mysql, mysql2
   and postgresql.
not formatted well http://how-we-work.bigbinary.com/conventionandstyle/parameter-indentation.html