Pymsl

Changelog:
- Updated for new MSL API syntax
- `load_manifest()` now takes the viewable ID as a single integer; no more multiple manifest requests in one HTTP request.
- `get_license()` now takes a single challenge and session ID passed as positional arguments respectively.
- `extra_manifest_params` kwarg added: read the documentation for more info.

Added license acquisition and updated documentation.

Supports loading manifests and returning them, but no license acquisition support yet.