Add minimumTrackTintColor and maximumTrackTintColor to SliderIOS

Summary:
There are still many other props that can be added to further customize the SliderIOS component, but I had a specific need for these two so I just went ahead and added them.
Closes https://github.com/facebook/react-native/pull/799
Github Author: Brent Vatne <brent.vatne@madriska.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Components/SliderIOS/SliderIOS.js

@@ -51,6 +51,18 @@ var SliderIOS = React.createClass({
      */
     maximumValue: PropTypes.number,
 
+    /**
+     * The color used for the track to the left of the button. Overrides the
+     * default blue gradient image.
+     */
+    minimumTrackTintColor: PropTypes.string,
+
+    /**
+     * The color used for the track to the right of the button. Overrides the
+     * default blue gradient image.
+     */
+    maximumTrackTintColor: PropTypes.string,
+
     /**
      * Callback continuously called while the user is dragging the slider.
      */
@@ -81,6 +93,8 @@ var SliderIOS = React.createClass({
         value={this.props.value}
         maximumValue={this.props.maximumValue}
         minimumValue={this.props.minimumValue}
+        minimumTrackTintColor={this.props.minimumTrackTintColor}
+        maximumTrackTintColor={this.props.maximumTrackTintColor}
         onChange={this._onValueChange}
       />
     );

React/Views/RCTSliderManager.m

@@ -50,5 +50,7 @@ static void RCTSendSliderEvent(RCTSliderManager *self, UISlider *sender, BOOL co
 RCT_EXPORT_VIEW_PROPERTY(value, float);
 RCT_EXPORT_VIEW_PROPERTY(minimumValue, float);
 RCT_EXPORT_VIEW_PROPERTY(maximumValue, float);
+RCT_EXPORT_VIEW_PROPERTY(minimumTrackTintColor, UIColor);
+RCT_EXPORT_VIEW_PROPERTY(maximumTrackTintColor, UIColor);
 
 @end