fix variable names and new zindex variables
add border-radius variables for toolbar and masthead
remove Twitter
CONTRIBUTING: reflect move away from *-wip branch style of development

BS3 commit 63f8c8caabe74159e85aafc1c244837850b8103c
links to a release instead of master, so rephrase description accordingly

BS3 commit 295fef252ecfcad784e04e752c6dedb873af4726
fix 9915

BS3 commit d666a89621ff85612390b3a0587e687eaae21060