chore(travis): fetch git tags before deploy

 As a workaround to try to avoid Travis 'tag_name not well formed'
 problem when deploying, we'll fetch git tags before doing so.
 For reference, see travis-ci/travis-ci#2510.
docs(contributing): create a contribution guide

Close #60
feat(overlay): add new mixin .make-overlay()

As the current offcanvas overlay is likely to be used in other places
as well, we're moving it to a separate mixin, with an optional position
argument.
refactor(overlay): ensure it uses display: block
feat(overlay): create the overlay component
fix(docs): don't overflow content in examples
chore(package.json): update dependencies to latest versions
chore(bower.json): set homepage

Bower warns that we don't have a homepage set in our manifest.
So, let's do this to make it happy :)
chore(scripts): inform when no tag available

The test is based in the $TRAVIS_TAG env var, made available
by Travis only when running in a tagged commit.
feat(navbar): create the static navbar style

Closes #58