WIP: add nosetests for plot elements

I want to have basic nosetests for ``fig_to_dict`` (which is the main driver of the d3 conversion process), comparing inputs to expected outputs. Combined with the JS tests in #132, I think this will do a pretty good job of covering the package.

Once these tests are in place, I'm going to try to modify the code for single codebase Python 2/3 support.