Add SparseSeries.to_coo method, a single test and one example.

This passes locally run nosetests but is failing on Travis. I think the latest Travis CI changes might have caused failures. I was able to get master to pass on Travis last week but now it is failing. 
I'm finding it a little hard to follow this example. Could you try constructing the matrix explicitly here, instead of using all the nan assignments below? It would probably also help if the matrix had fewer elements (I would go as small as possible).