overhaul grid mixins and variables, standardize fluid grid system and enable it across responsive layouts

fix form styles on prepend/append for consistency, update code docs with examples of the code snippets

- Button with dropdown menu.
- Small info button.
- Text alignment for navigation with no icon.
- Text input form with prepended icon
- Select input form with prepended icon

Updating to latest version of Glyphicons Halflings set (v.1.5) along with some icon enhancements like:

- Updated the Gylphicons Halflings set to version 1.5 (30 new icons)
- Optimized the sprite PNG files for minimum filesize without any quality loss
- Made some fixes for horizontal alignment at the LESS/CSS level (tags, bold, arrow-up, minus, chevron-down)
- Made some fixes for vertical alignment at the LESS/CSS level (chevron-down, resize-vertical, resize-horizontal)
- Renamed the old "calendar" class as "list-alt" class (because now we've a real calendar icon)
- Changed the "background-position" of the icon class to "14px 14px", so that a wrong class name will not be showing the glass icon and instead will show a blank icon (which is useful for just positioning the text after the icons in some cases)
- Added new icons (classes) to the base-css Docs (to both HTML file and Mustache template)
- Changed one of the button examples to danger style and used a white icon
- Renamed the sprite image files to save a few bytes here and there (from glyphicons-halflings-sprite.png to lyphicons-halflings.png and from glyphicons-halflings-sprite-white.png to glyphicons-halflings-white.png)

move around some comments in buttons.less, update labels to use correct variables, add snippet to docs about IE9

overhaul frontpage jumbotron, add js for twitter buttons back in, fix up some form ids and classes, straighten out the use of primary button variable

move css builder and downloader to dedicated page, start to update remaining sections on LESS docs page