Add a helper function to read in all planets into a table like structure.

The OEC uses XML for its data, which is great due to the flexibility
as well as the accessiblity but XML is hard if you just get the data.
This pull-request introduces a helper function which reads in all the 
planets data into a [pandas](http://pandas.pydata.org/) dataframe.

This allows really easy exploration of the planet data as well as 
makes it easy to plot it using python tools like matplotlib.
http://nbviewer.ipython.org/gist/dawehner/e59a4fd17645ca383902
provides an example how to use it.