WIP: fixes to rolling_window functions with center=True
#3155, #2953

@bmu, still interested in your help on testing this.
 Just updated my fork of pandas. How can I pull your branch into my repository?
 ```
git remote add y-p https://github.com/y-p/pandas.git; 
git fetch y-p; 
git checkout y-p/center_window -b center_window
```
 thanks, will post my comments in #2953.
 push to 0.14 for further review, amazing this is mergeable.....
 guess moments code hasn't been touched in a while
 The tests suite for this was a nightmare to get working, and so I've
never trusted these changes. Throwing the issue back in the queue.
