NEW: GridField can sort on relation columns

Visual example. News `ModelAdmin` section where each article has_one author:

![screenshot from 2013-10-23 22 01 52](https://f.cloud.github.com/assets/1655548/1394257/9945dffe-3c26-11e3-9cdd-c7ea0e42a400.png)
![screenshot from 2013-10-23 22 02 18](https://f.cloud.github.com/assets/1655548/1394258/9c3545c4-3c26-11e3-8d8a-5e997d3c6af2.png)