chore(.gitignore): ignore release .zip files

For some reason, they weren't ignored. This caused the latest
release build in Travis to wipe those files, so we couldn't upload
them to the GitHub Release.
See https://travis-ci.org/injoin/frontkit/builds/34651483#L763
feat(navbar): define bg for nav link on hover

Added @navbar-link-hover-bg variable that allow us to define a
background color for a nav link on hover.
chore(icons): update to the latest frontkit-font

Dropping the old, bad WHH Glyphs font.
feat(docs): add social network buttons

Added GitHub forks/stars and Twitter follow/tweet buttons.
Also, for easier maintenance of such buttons, I created a Swig
macro for them.

Closes #56
docs(readme): use flat-square badges style
chore(package.json): update dependencies

grunt-jscs@0.7.1 and grunt-karma@0.9.0
docs(tooltip): indicate that JS is required
refactor(tooltip): offset the tooltip from its target
chore(release): release 0.3.1
Merge branch 'master' into gh-pages
chore(release): docs release v0.3.1