feature(md-icon-button): set fixed line height for wrapped content (#1469)

`<md-icon></md-icon>` should be centered inside `<button md-icon-button></button>` when it's size is reduced in additional fix it for md-fab

Closes #1426

Author: andrewmnlv

src/lib/button/_button-base.scss

@@ -112,5 +112,6 @@ $md-mini-fab-padding: 8px !default;
 
   i, md-icon {
     padding: $padding 0;
+    line-height: $md-icon-button-line-height;
   }
 }

src/lib/button/button.scss

@@ -43,6 +43,7 @@
   border-radius: $md-icon-button-border-radius;
 
   .md-button-wrapper > * {
+    line-height: $md-icon-button-line-height;
     vertical-align: middle;
   }
 }