Add license terms file

Initiated by https://github.com/ipython-contrib/IPython-notebook-extensions/issues/75, this adds a stripped version of the IPython license file.
@Carreau, @damianavila Comments ?

To the contributors to this repo:
@bjanssen
@davidovitch
@dbarbeau 
@ibell 
@kinverarity1 

Do you agree to using the same license model (modified BSD license) as IPython for this repository ?