Add a custom data source to examples folder

Add an example of how to set up a custom data source and using it via a simple trading algorithm in zipline. 

I have had a few trouble trying to learn how to integrate zipline with a quotes database or 3rd party historical quotes API; or figure out how to manipulate a data source to generate other information like option greeks to send down to handle_data(...) of TradingAlgorithm(...). So wanted to write a simple cookbook example for others who want to do the same thing. 

You can simply run the file:
python custom_data_source.py 

It should make a custom data source generating random quotes on minute bars; and buy 1 share of AAPL and plot the results.