Zipline beginner tutorial

First draft of a tutorial for zipline.

Depends on https://github.com/quantopian/zipline/pull/357.

View the tutorial in all it's glory here:
http://nbviewer.ipython.org/github/quantopian/zipline/blob/0ba07e6efbc41db7eb70c7a269142fcf4b988698/docs/tutorial.ipynb