Add new Getting Started page

Modified content in start-en.hbs to match the new theme, 
Added page specific css in content/_base,scss
Modified colours in main/_base.scss and footer/_base.scss
Modified link colours in _variables.scss 
Should revisit the wording in the first sentence. Seems a little odd to mention using WET on a site to build sites and applications. Maybe drop "on your own site" to resolve the awkward wording.