[ #1660] Add support for project documentation in docviewer plugin

Same as https://github.com/playframework/play/pull/580 but hopefully with right commit this time :)

+ renamed `index.html` to `template.html`

I've extended docviewer plugin to support project documentation in textile files in $app/documentation folder. The same way it works for play documentation. You can open it by accessing @projectdocs url. You can also generate zip file with static project documentation htmls generated from textile using play doc:export command.

Issue: #1660

http://play.lighthouseapp.com/projects/57987-play-framework/tickets/1660-Add-support-for-project-documentation-in-docviewer-plugin