fix(dialog): add border radius (#1872)

Fixes #1868.

Author: crisbeto

src/lib/dialog/dialog-container.scss

@@ -2,11 +2,13 @@
 
 
 $md-dialog-padding: 24px !default;
+$md-dialog-border-radius: 2px !default;
 
 md-dialog-container {
   @include md-elevation(24);
 
   display: block;
   overflow: hidden;
   padding: $md-dialog-padding;
+  border-radius: $md-dialog-border-radius;
 }