Removing margins from buttons.

Components should not define margins around themselves, as it
breaks encapsulation.

src/button/_button.scss

@@ -27,7 +27,6 @@
   height: $button-height;
   min-width: $button-min-width;
   padding: 0 $button-padding;
-  margin: 0 $button-margin;
   display: inline-block;
   @include typo-button();
   overflow: hidden;