update Installation webpage to include ActivePython distribution
#### Language to Add

on http://pandas.pydata.org/pandas-docs/stable/install.html#installing-pandas

Please add:

> **Installing pandas with ActivePython**
> Installation instructions for ActivePython can be found [here](https://www.activestate.com/activepython/downloads).

Under "Installation," please edit this sentence to read:

> Instructions for installing from source, PyPI, ActiveState, various Linux distributions, or a development version are also provided.

(Please feel free to make changes to my suggested language above; the important part is including the link to the ActiveState distro.)

#### Problem description

As part of ActiveState's sponsorship of NumFOCUS, they have asked that the ActivePython distribution be linked from the pandas package page. (This was approved by @jreback in conversation with Leah.)

It would be fantastic if we could get this update made by the end of February.

Thanks so much,
Gina
