Refactor from single module to package

[davidism]

Closes #106

Only names present in the API documentation prior to the split are still importable from the top-level itsdangerous name. If anyone was relying on undocumented internals, they'll need to fix their imports. Eventually I only want to expose the Serializer classes at the top-level since that's what most users would interact with, with everything else imported from its actual location.

Deciding what submodules to create was more difficult than I expected. I'm happy with the organization I came up with, but there's still time for input if you think anything should be different.

src/itsdangerous
├── _compat.py
├── encoding.py
├── exc.py
├── __init__.py
├── _json.py
├── jws.py
├── serializer.py
├── signer.py
├── timed.py
└── url_safe.py

Todo:

• code
• tests
• docs