Generalised game support, masterlist URLs are now part of game settings.

Part of issue #3, #17 work. Paths aren't getting written to settings
file on exit yet though, and there's no GUI for editing the settings yet
(besides existing URL settings). Changes to libespm.yaml are just
guessing that FO3 and FONV use the same settings as Skyrim.
Changed some metadata member types, constructors.

Message types, languages are now given by int constants for simpler
comparison. Tag constructor now explicitly takes addition/removal flag.
Removed unused metadata member functions. Also removed CRC/isActive calc
from Plugin constructor, and implemented YAML emitter for Game objects.