Custom NSURLProtocol to auth fetching private site content.

We can fetch images from private sites courtesy of a bearer token added to the request header, but images loaded in a `UIWebView`'s content are tricky.  This patch introduces a `NSURLProtocol` subclass to add the header and make the request. 
Refs: https://github.com/wordpress-mobile/WordPress-iOS-Editor/issues/115

### Before
![ios simulator screen shot nov 19 2014 5 19 32 pm](https://cloud.githubusercontent.com/assets/1435271/5116868/779e4874-7013-11e4-8306-03c5f29c9be3.png)

### After
![ios simulator screen shot nov 19 2014 5 33 48 pm](https://cloud.githubusercontent.com/assets/1435271/5116871/7de1dcbe-7013-11e4-9b0b-624560ec7c90.png)

> it should also work with the non-mapped domain and https, even though we'll need to do some URL rewriting, or change how the preview URL is constructed

:+1: