Add SegmentedControlIOS

Summary:
Fixes #534:

![screen shot 2015-03-31 at 7 52 10 pm](https://cloud.githubusercontent.com/assets/153704/6934038/742ddd34-d7e3-11e4-8f55-3eb7d9d3f1cd.png)

```jsx
<SegmentedControlIOS
  tintColor="#ff0000"
  values={['One', 'Two', 'Three', 'Four']}
  selectedtIndex={0}
  momentary={false}
  enabled={true}
  onValueChange={ (value) => console.log(value) } />
```

This only supports string-based segments, not images. Also doesn't support full customization (no separator images etc); I figure this is a good MVP to lock-down a basic API

I also included a snapshot test case, but the images keep coming out funky. When I look at the sim, I see that the text labels show up for the selected segment, but the snapshot keeps coming out with no text on those segments. I tried forcing a delay, but same result. Is that explainable?

Obviously happy to change anything about the API, code-style nitpicks, etc
Closes https://github.com/facebook/react-native/pull/564
Github Author: Clay Allsopp <clay.allsopp@gmail.com>

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

Examples/UIExplorer/UIExplorerList.js

@@ -43,6 +43,7 @@ var COMPONENTS = [
   require('./NavigatorIOSExample'),
   require('./PickerIOSExample'),
   require('./ScrollViewExample'),
+  require('./SegmentedControlIOSExample'),
   require('./SliderIOSExample'),
   require('./SwitchIOSExample'),
   require('./TabBarIOSExample'),