Check for external nif before loading.
Created detailed requirements list in character files.

This ticks serialization version to 3. Not really a compatibility-breaking change, but future versions will rely on the new data being there to notify players of missing mods. Added code to upgrade the files if they are behind based on the current setup. This should be generally safe because not many people are trading files yet.
Version tick and upgrade code for new version.

This should close #66 and pave the way for implementing #6 at last.
Prevent missing race from hanging the character script.

If a character's race is missing (usually because it's a custom race provided by a mod) it would spend a very long time try to update before finally giving up. Now if the race is missing it is set to NornRace and the script only tries LoadCharacter twice.