Changed 'illustration_builder.py' to '__illustrate__.py' everywhere.

FILESYSTEM: you should rename any illustration_builder.py files in your
material packages to __illustrate__.py instead.
Example script for previous commit.

    #! /usr/bin/env python
    import os

    for directory_path, subdirectory_names, file_names in os.walk('.'):
        for file_name in file_names:
            if file_name == 'illustration_builder.py':
                file_path = os.path.join(directory_path, file_name)
                new_file_path = os.path.join(directory_path, '__illustrate__.py')
                command = 'git mv {} {}'
                command = command.format(file_path, new_file_path)
                #print command
                os.system(command)

That script will rename files everywhere in a filetree.
Merged master.