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.
We need it because it will get added to generated database.yml as follows:

``` ruby
<% git_branch = `git symbolic-ref HEAD 2>/dev/null`.chomp.sub('refs/heads/', '') %>
<% repository_name = `git rev-parse --show-toplevel`.split('/').last.strip %>
```

And then we can use those variables dynamically for calculating database name in database.yml

```yaml
development:
  adapter: postgresql
  database: <%= "#{repository_name}_development_#{git_branch}"[0...63] %>
  encoding: unicode
  host: localhost
  pool: 5
  username: postgres
  password: 
```