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)

I'm thinking of a somewhat elaborate attack, but this could be a security issue.

Let's say I have a wp.com blog with a mapped domain (_target.domain_), mapped to _target.wordpress.com_. And request.URL is `http://target.domain/whatever`. Being a mapped domain, the request can't use HTTPS and can be intercepted.

An attacker could use DNS spoofing or other techniques to make your device think that `target.domain` points to the attacker server. Since it doesn't use HTTPS, there's no extra validation, and your wp.com OAUth2 token would be sent to a third party server.

I would limit this to requests going to `.wordpress.com` and using https, which is should work for images, even when the site has a mapped domain