Plotly Renderer with Accompanying Test Function

The renderers package now includes a plotly subpackage which contains the plotly_renderer module and the plotly_utils module.

The plotly_renderer module can be tested with the test_plotly module now in the tests package. It is very similar to the test_basic module in that a user can call a function that creates a matplotlib plot, instantiates a Renderer, instantiates an Exporter with said Renderer, and then calls the Exporter.run() method. The assertion used in test_basic is just modified to fit the different data set, but it is still used.

A public username and api_key are stored in the test_plotly.py file so that the function, plot_sin, can be called without needing to change any files.

Thanks for kicking the mplexporter project off!