Target Windows Phone 8.1

I'm using Zeroconf in a Windows Phone application that I want to target Windows Phone 8.1 with. Zeroconf currently doesn't target WP8.1 and generated an error when I tried installing it, so I added a new project within the Zeroconf solution that targets 8.1. 
I also changed the NuGet package spec so that users can add the package to their WP8.1 project. 

Verified locally that the package works and that no underlying source code changes are required. 
Example: Enumerating machines without regression in WP 8.1 project (still works as you'd expect)
![image](https://cloud.githubusercontent.com/assets/1258037/3557455/8b91d972-092f-11e4-8419-0143186b87c5.png)

Package description now says it also targets 8.1:
![image](https://cloud.githubusercontent.com/assets/1258037/3557472/ccb2856e-092f-11e4-8754-63b493a399d8.png)