Mixtures and __repr__ enhancements.

[galenseilis]

• Added a brief docstring for the file, although it should probably be expanded on in the future.
• Changed order of imports to match PEP8.
• Adjusted repr so that brackets are given with the distribution name.
• Added a mixture distribution class.

[galenseilis]

Oops, I forgot to import type hints:

from typing import List, NoReturn