Set onScroll handler on tab bar view (ptomasroos#1005)

* adds passing onScroll to ScrollView in ScrollableTabBar

* adds throttle to onScroll events of the underlying ScrollView

Author: frobertson-ccb

ScrollableTabBar.js

@@ -164,6 +164,10 @@ const ScrollableTabBar = createReactClass({
       width: this.state._widthTabUnderline,
     };
 
+    const {
+      onScroll,
+    } = this.props;
+
     return <View
       style={[styles.container, {backgroundColor: this.props.backgroundColor, }, this.props.style, ]}
       onLayout={this.onContainerLayout}
@@ -176,6 +180,8 @@ const ScrollableTabBar = createReactClass({
         directionalLockEnabled={true}
         bounces={false}
         scrollsToTop={false}
+        onScroll={onScroll}
+        scrollEventThrottle={16}
       >
         <View
           style={[styles.tabs, {width: this.state._containerWidth, }, this.props.tabsContainerStyle, ]}