Added Rails generator command: rails g haml:application_layout convert

PR for #47
https://github.com/indirect/haml-rails/issues/47

Usage
Once you install haml-rails gem, you can run the following command
to convert application.html.erb to application.html.haml.

`$ rails generate haml:application_layout convert`
or
`$ rails g haml:application_layout convert`
Merge pull request #73 from serv/master

Added Rails generator command: rails g haml:application_layout convert