Add aria-haspopup and aria-expanded attributes to dropdown plugin

1. Add aria-haspopup="true" and aria-expanded="false" to dropdown docs
2. Toggle aria-expanded between "true" and "false" when opening and closing dropdown

Satisfy item 1 from https://github.com/paypal/bootstrap-accessibility-plugin#dropdown. See #13553.
Add a bottom affixed element to affix visual tests.
Refactor determining affix state into a separate expanded method
  in order to handle multiple edge cases, specifically when the document
  height is dynamic.
Always reposition an affix that is affixed to the bottom.
Fix issue where bottom affixed element floats over the footer when the
  document height is smaller than the viewport height.
update to rouge v1.5.1 in Travis CI build
update to Jekyll v2.1.1 in Travis CI build
Update grunt-csscomb.
Run `grunt`.
Merge pull request #13873 from twbs/grunt-csscomb

Update grunt-csscomb
Update Blob.js to the latest version.
Update Filesaver.js to v2014-07-21.
Update all devDependencies.
Run `grunt`.
cleanup style
Merge branch 'dropdown-aria' of git://github.com/mckramer/bootstrap into mckramer-dropdown-aria

Conflicts:
	docs/_includes/js/dropdowns.html
Merge pull request #14208 from twbs/mckramer-dropdown-aria

Mckramer dropdown aria
Merge pull request #14207 from twbs/charlesbjohnson-affix-bottom-when-dynamic-height

Charlesbjohnson affix bottom when dynamic height