Organize python into sub-sections to be consistent with three.js organization

[abelnation]

This is an initial clean-up of the python code to separate it out into files. In the future, I'd like to take this further by:

• distinguishing between "core" three.js classes (i.e. classes that map directly to three.js classes) and "convenience" classes (i.e. classes that are more for easing the use of three.js in ipython)
• refactoring all enums out to a enums.py files

No classes were modified in this PR. They were simply moved to separate files. Necessary imports were added to each sub-file.

@jasongrout @SylvainCorlay

[abelnation]

Closing this PR, as I think this organization would change with auto-generation.