Add encoder to handle masked numbers.

@chriddyp , it'd be cool to get your input on this one, I think you wrote the encoders in here originally. I need to write a test before we pass this off still too.
It'd be cool if we kept all these X_imported variables in the same namespace. Like, attached to the session. We can refactor that later though. Checkout https://github.com/plotly/python-api/blob/master/plotly/tools.py#L28-39

It'd be nice if all of this info was in one spot.

Here's a way to check if it's imported without spending the time to import it too! We should eventually do this with all our optional modules on import (should take a negligible amount of time...)

http://stackoverflow.com/questions/14050281/how-to-check-if-a-python-module-exists-without-importing-it