Retrieve network adapter IP addresses, and show them on easyadmin page.

Added a convenience function, as requested in #693 and #968, for checking the local device's IP addresses.

It works by wrapping `ifconfig` on Linux/OSX, and `ipconfig /all` on Windows. It has been tested on Ubuntu, OSX, Windows XP, and Raspbian.

Exposed the IP addresses on the easyadmin page; could use some styling, but hopefully at least having it is an improvement:
![image](https://f.cloud.github.com/assets/618416/1647913/4afb1e64-595d-11e3-9ef6-2a1ed60df0f1.png)