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.
@prathamesh-sonpatki I think, we don't need this assignment `git_branch =`, the remaining part will work the same.