fix(menu): safari z-index fix for .menu .scroll-content

Closes ionic-team#1408

Author: Adam Bradley

demos/directive/sideMenus/simple/style.css

@@ -0,0 +1,7 @@
+---
+name: simple
+component: ionSideMenus
+---
+.menu-left .scroll-content {
+  background-color: #f4f4f4;
+}

scss/_menu.scss

@@ -17,6 +17,14 @@
   width: $menu-width;
 
   background-color: $menu-bg;
+
+  .scroll-content {
+    z-index: $z-index-menu-scroll-content;
+  }
+
+  .bar-header {
+    z-index: $z-index-menu-bar-header;
+  }
 }
 
 .menu-content {

scss/_variables.scss

@@ -669,6 +669,8 @@ $z-index-item-reordering:         9 !default;
 $z-index-item-toggle:             3 !default;
 $z-index-loading:                 13 !default;
 $z-index-menu:                    0 !default;
+$z-index-menu-bar-header:         11 !default;
+$z-index-menu-scroll-content:     10 !default;
 $z-index-modal:                   10 !default;
 $z-index-pane:                    1 !default;
 $z-index-popup:                   12 !default;