Convert gh-pages CSS to Sass.

 * https://github.com/blog/1867-github-pages-now-runs-jekyll-2-2-0
 * http://jekyllrb.com/docs/assets/
Build select2-bootstrap-css on Twitter Bootstrap v3.2.0.

Update development dependencies:

 * Twitter Bootstrap from v3.0.0 to v3.2.0
 * respond.js from v1.2.0 to v1.4.2
 * jlong/sass-twitter-bootstrap to twbs/bootstrap-sass

Adjust paths in Makefile, fix glitch when copying Bootstrap fonts to _jekyll.
Adjust paths in lib/build.scss, build.less.
Update Less patch.

Update readme - one known issue has been fixed in Bootstrap itself.
Fix Bootstrap version number on docs index page.
Link to select2-bootstrap.css in the bootstrap3 branch, not the one from master.
Replace Makefile tasks with Grunt tasks:

 * replace "make copy_docs_assets" with "grunt copy" (via grunt-contrib-copy)
 * replace "make serve_docs" with "grunt jekyll:build", "make build_docs" with "grunt jekyll:serve" (via grunt-jekyll)
 * replace "make pages" and "make pages_setup" with "grunt gh-pages" (via grunt-gh-pages); note that in contrast to "make pages", "grunt gh-pages" will not (yet) automatically push changes to origin
Add current version number to CSS files, update Bootstrap version.
Update less.patch.
Whitespace in readme.
Alter "grunt gh-pages" to immediately push changes.
Merge pull request #45 from fk/bootstrap3

Build select2-bootstrap-css on Twitter Bootstrap v3.2.0