Add basic Mook class

[jag1g13]

Mooks are able to engage in combat (test_mook_battle in test_mook.py).

Design decisions to make (may require changes):

• Should Mooks raise exceptions when they die?
• Should the deck be passed to all functions requiring a card flip, or should there be a singleton deck that all flips can draw from?
• Verify JSON schema (and add extra bits as they arise when going through source books)
• How to handle damage flips? Attacker makes them with knowledge of defence modifiers, or defender makes them with knowledge of attack modifiers?