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

Yes this is correct git usage :) 

The problem with this is that as soon as you do anything with the DataFrame you've created the column metadata is going to be lost. Probably the right way to do it would be to store the information _inside the column index_ itself. So it's a bit of an involved undertaking. I will take a closer look at your patch and think about how much work it would be to do 
 That makes sense. Let me know!
 Hey sorry this PR has been open for so long. I'm going to close it until more work is put into a deeper solution for tracking metadata. It's a pretty big project I think
