Play Framework templates

I created build:play rake task that compiles a play template version of govuk_template.

Changes: 
 - PlayProcessor - follows the TarProcessor's example and provides play specific template placeholders in the handle_yield method
- PlayPackager - follows the TarPackager's example and packages into a 'play_govuk_template-X.X.X.tar' and employs the PlayProcessor to perform erb processing.