Update interface to fix needs found by Hatchet.

[kisaacs]

This assumes the disambiguation of Hatchet-local vis wrapper stuff and the lower-level Roundtrip.

[cscully-allison]

• [x] Turn Roundtrip into a normal class
• [x] Define a function for loading arbitrary JS files
• [x] Define a function for loading arbitrary CSS files
• [x] Define a function for loading arbitrary HTML "template" files
• [x] Define a function for binding multiple jupyter scope variables to multiple javascript scope variables
• [x] Define a function for retrieving data from javascript code
• [x] Investigate a means to watch a Jupyter cell and update vis when that cell updates

[cscully-allison]

Got all this done and done :)