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. 
If you're not reusing this data for multiple tests, I think it is clearer if you put the code in or next to the relevant tests instead of up here.