Make rounded button groups work on actual buttons

Previously this only worked on anchor tags, however this should also work on actual buttons.
Merge pull request #1791 from christianv/make-rounded-buttons-work

Make rounded button groups work on actual buttons
Merge pull request #1793 from mattnjones/doc-fix

Fix code example to use @include instead of @import
Fixes #1722
updated split button docs
updated contributor doc
Merge branch 'master' of github.com:zurb/foundation
updated changelog
updated sass docs to talk about standalone
updated changelog
updated changelog
bump version to 4.0.5
Update index.html

Updated release date
Merge pull request #1802 from justinledwards/patch-1

Update index.html