Add rails_root, rails_env and migration_role as variables with sensible defaults.
Reliably set the default vars.
Merge github.com:capistrano/rails into add_rails_config_options

Conflicts:
	lib/capistrano/tasks/assets.rake
Updated README.md with new options.
Merge pull request #12 from craigmcnamara/add_rails_config_options

Add rails_env and migration_role as variables with sensible defaults.