`_send_to_plotly` now replaces a figure's '\n' with '<br>'.

In response to (5) in https://github.com/plotly/python-api/issues/46
@chriddyp, i wrote this such that `figure` doesn't get changed on the user, but it *isn't* strictly what gets sent to plotly... in addition. We also need to consider if this should happen anywhere else (e.g., static image generation?)