WIP: fixes to rolling_window functions with center=True

[ghost]

#3155, #2953

[ghost]

@bmu, still interested in your help on testing this.

[bmu]

Just updated my fork of pandas. How can I pull your branch into my repository?

[ghost]

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

[bmu]

thanks, will post my comments in #2953.

[jreback]

push to 0.14 for further review, amazing this is mergeable.....

[cpcloud]

guess moments code hasn't been touched in a while

[ghost]

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.