Move Key class to separate module

This also allows re-organising the imports a bit.
Provide pskc.encryption docstrings

This documents classes in the pskc.encryption module.
Provide pskc.mac docstrings

This also hides two properties that are not part of the public API.
Complete pskc.policy docstrings

Also contains small consistency improvement.
Provide pskc.parse docstrings

This documents most of the API of the parsing functions and the PSKC
class.
Provide pskc.key docstrings

This documents most of the information that is available per key and
adds a few other minor cosmetic changes.

This also re-organises the key properties to be in a slightly more
logical order and renames the userid key property to key_userid to more
clearly distinguish it from device_userid.
Move PSKC class to toplevel module

This also splits the parsing to a parse() function for consistency.