DO NOT MERGE: Split documentation in to sub-app

[edwardhorsford]

This PR moves documentation and examples outside of app and in to a new sub-app called docs. This gives us a more consistent place to put documentation, and means our upgrade path is easier - users upgrading via copying app will now be able to get up to date documentation.

This PR also introduces a new documentation 'home' - the intention being we can push this to heroku and have a single place we can point people at to get the kit and docs, etc.

You can play with it here: http://prototype-kit-documentation.herokuapp.com/

Fixes #151

Changes:

• Move docs and examples in to sub-app /docs/
• New docs start page which links to everything.
• Uses github API to get the url to the zip of the latest release - easier than linking to github.
• Renders markdown files as html - markdown will still work and link in Github though.
• Introduces a config var to enable docs or not.
• Introduces a env var to redirect the root to docs - this means we can deploy to govuk-prototype-kit.herokuapp.com and have the docs be the primary page.
• Change to npm module nunjucks from express-nunjucks, as it didn't seem to correctly support sub-applications.

Things that aren't finished:

• I couldn't fully split out the sass it needs. Ideally it would be included via grunt, but it needed an include in the app folder. Would be better if it produced it's own sass file that was an extra include for the docs templates only.
• Most app middleware isn't applied to the new docsApp - however this doesn't seem to have any detrimental effect.
• Routing in the sub-app might have a bug - I couldn't get redirect routing relative to the mount point to work.

NB: I'd expect this to be a major version change.

[joelanman]

this should get converted to lowercase like the other environment booleans

[edwardhorsford]

I think I've addressed all your comments except caching of the url.

[edwardhorsford]

@joelanman I've implemented caching of the url for the life of the app. This is probably fine as long as the app goes to sleep every so often. If we ever have persistently running apps, we may want to do something a bit more clever.

[timpaul]

'About the prototype kit'?

[gemmaleigh]

Can this be closed in favour of #244? @edwardhorsford