DOC: add nlargest/nsmallest to API.rst
At the moment there's no easy way to partially sort the DataFrame to get top N rows, could be useful feature.

There's partial sorting available for numpy within the [bottleneck](http://berkeleyanalytics.com/bottleneck/reference.html#bottleneck.partsort) library.

Here's link to SO question about numpy partial sort - http://stackoverflow.com/questions/10337533/a-fast-way-to-find-the-largest-n-elements-in-an-numpy-array
