ENH: Add window to exponentially weighted moment functions
Currently there is no way to specify a rolling window for the exponentially weighted functions. From the [docs](http://pandas.pydata.org/pandas-docs/stable/computation.html#exponentially-weighted-moment-functions) the weighting function is giving by

![eq1](https://cloud.githubusercontent.com/assets/2401026/10417648/45d29b66-7011-11e5-97b6-2f0a137f86eb.png)

However I am wondering if it is straightforward to add a _window_ parameter, similar to the rolling window statistics. This would change the formula to look like

![eq2](https://cloud.githubusercontent.com/assets/2401026/10417649/4c34e6d0-7011-11e5-97bb-b99b8c7677f5.png)
