Add IB_DESIGNABLE to Switch to allow it to show up in IB.

Test Plan:
Add an MDCSwitch to a storyboard.  MDCSwitch should render in Interface Builder / Xcode.
Add an MDCSlider to a storyboard.  MDCSlider should render in Interface Builder / Xcode.

Reviewers: ajsecord, #material_components_ios_owners

Reviewed By: ajsecord, #material_components_ios_owners

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D127

Author: Ian Gordon

components/Slider/src/MDCSlider.h

@@ -35,6 +35,7 @@
 
  @ingroup Slider
  */
+IB_DESIGNABLE
 @interface MDCSlider : UIControl <NSCoding>
 
 /**

components/Switch/SwitchAutolayout/Base.lproj/Main.storyboard

@@ -27,7 +27,7 @@
                                 <rect key="frame" x="276" y="63" width="51" height="31"/>
                             </switch>
                         </subviews>
-                        <color key="backgroundColor" red="0.90196079019999997" green="0.90196079019999997" blue="0.90196079019999997" alpha="1" colorSpace="calibratedRGB"/>
+                        <color key="backgroundColor" red="1" green="1" blue="1" alpha="1" colorSpace="calibratedRGB"/>
                         <constraints>
                             <constraint firstItem="6y5-Bf-Aj4" firstAttribute="top" secondItem="eWK-42-F2X" secondAttribute="bottom" constant="8" id="7iu-1j-2cK"/>
                             <constraint firstItem="eWK-42-F2X" firstAttribute="top" secondItem="y3c-jy-aDJ" secondAttribute="bottom" constant="8" id="Ovn-uT-lwy"/>

components/Switch/src/MDCSwitch.h

@@ -28,6 +28,7 @@ CG_EXTERN CGSize MDCSwitchSizeThatFits(CGSize size);
  *
  */
 NS_CLASS_AVAILABLE_IOS(7_0)
+IB_DESIGNABLE
 @interface MDCSwitch : UIControl <NSCoding>
 
 // TODO(iangordon): Unify our Color handling over all of our components