Make button border style configurable

Author: andischerer

Diff for: scss/_variables.scss

@@ -93,8 +93,8 @@ $button-bar-button-height:        32px !default;
 $button-bar-button-padding:       8px !default;
 $button-bar-button-icon-size:     20px !default;
 
-$button-default-border:       transparent;
-$button-default-active-border:null;
+$button-default-border:           transparent !default;
+$button-default-active-border:    null !default;
 
 $button-light-bg:                 $light !default;
 $button-light-text:               #444 !default;