Change post owner functionality

Some notes:

 - Owner changes are logged as post revisions.
 - The client UI won't let you change ownership of posts if you have mixed authors selected, but the backend has no such restriction.
 - The spec is a bit lacking.

![image](https://cloud.githubusercontent.com/assets/627891/2552397/ce8e8a2a-b698-11e3-8ffc-cd8106258f15.png)
![image](https://cloud.githubusercontent.com/assets/627891/2552408/e7efc45c-b698-11e3-9a8a-2346eb10c219.png)
![image](https://cloud.githubusercontent.com/assets/627891/2552413/f0f83f98-b698-11e3-88a5-7ce20d3797ff.png)