Add support for project documentation in docviewer plugin

I've extended docviewer plugin to support project documentation in textile files in <code>$app/documentation</code> folder. The same way it works for play documentation. You can open it by accessing <code>@projectdocs</code> url. You can also generate zip file with static project documentation htmls generated from textile using <code>play doc:export</code> command.

Issue: #1660

http://play.lighthouseapp.com/projects/57987-play-framework/tickets/1660-Add-support-for-project-documentation-in-docviewer-plugin