Disabled test t274 for the time being; my changes screwed up exception handling.
Added some useful methods for Python/JS interaction.
Added globals (working) and locals (not working) builtins.
Added format method to string.
A slew of changes to support yield/resume for browser-based execution and async call handling.
A first cut at Python's urllib.request module. Basically only supports urlopen right now, but it's enough for students to get data from web services.