Create gem scaffold

* Generated from bundler `bundle gem govuk_admin_template`
Fill in gem details
Use rails engine
Pass bootstrap mixins and styles through

* By including govuk_admin_template in application.js the bootstrap
javascript is passed through
* By including govuk_admin_template in application.scss the bootstrap
styles and mixins are passed through
Add IE javascript shims for HTML5 and bootstrap 3

* Precompile scripts
* Create basic layout with conditional IE scripts and non-javascript
helper class
Yield app content

* Use standard head, content and page_title yields as used in
govuk_template