Add HoganJS 3.0.0

NPM only holds 2.0.0 so include the core files from this version:

https://github.com/twitter/hogan.js/commit/b1328c06eedf316bbe4c54b189ecb47cff7ac2ed
Add readdir package
Add Hogan-middleware

Contains following changes to NPM version:

* all file watching removed
* compiles templates from additional views folder
Add Grunt task to convert template

Converts to proposed version of mustache that supports inheritance.
Add new version of template
Bump govuk_template to 0.6.2

Now includes headerClass variable.
Add supporting changes for inheritance

* make nodemon watche html files.
* add missing assetPath variable to each view
Make index.html work with inheritance tags

* add govuk_template as the layout
* set the propositionHeader block to the propositional_navigation
  partial
* set the page content as the content block
Move local stylesheet to partial & tidy up
Merge pull request #4 from tombye/add-template-inheritance

Add template inheritance