Fixes 1069: Make content in "Make a Link" screen narrower on iPad

Fixes #1069 
In WPAlertView, if the device is an iPad, adjust the backgroundView's frame and autoresizingMask. If the device is an iPhone/iPod touch the backgroundView setup logic will function as it did previously. Some screen shots of the end result: 

![link_fixed_portrait](https://f.cloud.github.com/assets/154014/2211079/1f4e95fa-99a1-11e3-8c77-4515b6875fe6.png)
![link_fixed_landscape](https://f.cloud.github.com/assets/154014/2211082/296bdda4-99a1-11e3-9444-65272aaefdca.png)

This code also works with the WPAlertView's side-by-side mode:
![link_fixed_landscape2](https://f.cloud.github.com/assets/154014/2211097/4a9be49c-99a1-11e3-8ccb-e8f590fbe098.png)
![link_fixed_portrait2](https://f.cloud.github.com/assets/154014/2211092/4137696c-99a1-11e3-980c-c3a209ae2a82.png)

In addition, I addressed another issue where the UILabels that preceded both text fields are now hidden if both of the labels' text property are blank (this is a "before PR" screenshot FYI):

![link_not_fixed_portrait-2](https://f.cloud.github.com/assets/154014/2211161/11554bdc-99a2-11e3-98de-80f13e031a4e.png)