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 
Customizing WET isn't developing for WET. Also "design a new WET theme" is ambiguous. Probably should use "design a new theme for WET" instead (since the "WET theme" is the theme used on the WET website). Also not sure designing a new theme for WET counts as developing for WET, since the new theme may not be contributed back to WET (seems more like extending WET or customizing WET).