Navigator.NavigationBar Landscape Fix

Summary: NavigationBar items are fixed to Portrait when Device is in Landscape.

This supplements that fix by removing the `width` properties and just using `left` and `right` positioning respectively.

<strong>Before:</strong>

![ios simulator screen shot sep 8 2015 1 27 02 pm](https://cloud.githubusercontent.com/assets/755943/9743817/3e6b858a-5636-11e5-80e8-81e62b46c46e.png)

<strong>After:</strong>

![ios simulator screen shot sep 8 2015 1 29 21 pm](https://cloud.githubusercontent.com/assets/755943/9743822/43e7d4b4-5636-11e5-8e1c-9f13bdc492b2.png)

Closes https://github.com/facebook/react-native/pull/2606

Reviewed By: @vjeux

Differential Revision: D2426942

Pulled By: @ericvicenti