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.
`git_branch` and `repository_name` will get written to database.yml here https://github.com/bigbinary/handy/blob/add-database-yml-rake-task/lib/handy/add_database_yml.rb#L69-L70