implemented description property
Hi,

This is a basic implementation of description and unit properties in DataFrame. 
https://github.com/wesm/pandas/issues/39

I used dict to store both description and unit. I thought about using tuples but dict would be better if a user wants to modify individual descripition/unit. 

One can directly access description/unit, and also DataFrame.info() will print out those if they exist.

I haven't touched at the storing part yet so it is very rudimentary now (no way to save the descriptions), but I wanted to know what you think. Please let me know.

And also, is this (topic branch) right way to submit a pull request? I'm still learning about git. :)

Happy new year!
-Joon
