Bug 893707 - [bb][refactor] Headers

Main points:
* Spliited headers.css in to 6 new files: layout.css, icons.css, responsive.css skin.default.css, skin.dark.css, skin.organic.css
* Refatorized selectors and some rules re-arrangment
* Icons no longer needs extra markup
* Added responsive.css with rotation support
* Removed un-used images
* Removed "header with form" case as in no longer supported