split $tabbar-height into $tabbar-height and $tabbar-icon-width

by splitting tab bar height in height and icon-width (respectively for height and width settings related to the tab bar and tab bar sections), you can have wider icons/clickable areas in the tab bar, controllable via _settings.scss
$tabbar-icon-width defaults to $tabbar-height for a square icon

in below image I use icon fonts and a double width in a tab bar (this allows me to have a more meaningful icon combination

![foundation](https://f.cloud.github.com/assets/4661657/1861416/213ccab8-77c2-11e3-8f46-cd0a0ad683c6.png)

Signed-off-by: Maarten Ottens <m@maartenottens.com>