Merge pull request #2973 from LaurentGoderre/v4.0

HACK: Temporary fix for assemble issue 139

https://github.com/assemble/assemble/issues/139
Build: Add CSSMin task

- Minify all CSS on build
- Move all minification to a "minify" task
Build: refactor targets

Separate out chained task to more atomically related actions
Added Font logic for porting fonts from Bootstrap lib
Merge pull request #2976 from masterbee/hotfix--fontmapping

Added Font logic for porting fonts from Bootstrap lib
Markup: Changed aria-labelled-by typo to aria-labelledby (#2968).
Merge pull request #2977 from pjackson28/fixmarkup

Markup: Changed aria-labelled-by typo to aria-labelledby (#2968).
Merge pull request #2974 from nschonni/v4.0-add-cssmin

V4.0 add cssmin
Corrected missing polyfill resources -
* Excanvas and selectorvizr was missing from polyfill directory. Since yepnope has a 10000 millisecond resource timeout, this created a 20000 millisecond lag for all other tests and in Modernizr
* Corrects the lag notice by @pjackson in PR #2978
Merge pull request #2980 from masterbee/hotfix--missingpolyfils

Corrected missing polyfill resources -
Jaws polyfill: Added polyfill to fix the Jaws sectioning elements/heading level incrementation bug and the sectioning element/heading association bug.
Merge pull request #2978 from pjackson28/jawsariapolyfill

Jaws polyfill: Added polyfill to fix the Jaws sectioning elements/heading level incrementation bug and the sectioning element/heading association bug.
Jaws aria polyfill: jQuery style guide fixes
Merge pull request #2981 from patheard/jawsariafixes-jquery-style

Jaws aria polyfill: jQuery style guide fixes
Added 3 new helpers additions to data-ajax functionality
* data-ajax-append : appends the URL contents to the element
* data-ajax-before : put the URL contents before the element
* data-ajax-after : puts the URL contents after the element

Optimizations:
* removed the data-fetch into it's own event to allow for uniqID generation of ajaxed content to prevent collisions
* function now available to all elements that require ajax-fetching
* added a misc glod in the coffeescript to bring over .txt files. This could be improved but is required for ajax files
Merge pull request #2984 from masterbee/plugin-data-ajax

Added 3 new helpers additions to data-ajax functionality
Added $(':tabable') expression helper to assist with keyboard navigation targetting
Jaws ARIA polyfill: Added support for nested heading elements
Merge pull request #2989 from pjackson28/tweakjawspolyfill

Jaws ARIA polyfill: Added support for nested heading elements
Renamed megamenu plugin to something more generic
* Added sample and refined navigation model to better align with ARIA proposed
* Introduced a new sectional based navigation structure that leverage HTML5 a bit more with in page anchoring a navigational aids
* Still a bit buggy in fringe cases of mouseleave events, but exploring the issue