Add screen.getPrimaryDisplay() API.

spec/api-screen-spec.coffee

@@ -5,5 +5,12 @@ describe 'screen module', ->
   describe 'screen.getCursorScreenPoint()', ->
     it 'returns a point object', ->
       point = screen.getCursorScreenPoint()
-      assert typeof(point.x), 'number'
-      assert typeof(point.y), 'number'
+      assert.equal typeof(point.x), 'number'
+      assert.equal typeof(point.y), 'number'
+
+  describe 'screen.getPrimaryDisplay()', ->
+    it 'returns a display object', ->
+      display = screen.getPrimaryDisplay()
+      assert.equal typeof(display.scaleFactor), 'number'
+      assert display.size.width > 0
+      assert display.size.height > 0