Got a big chunk of battle working

Known issues:
1. If the player kills a monster, that monster's HP gets changed
forever. If you bump into it again, it'll have negative HP or 0 HP.
2. Player should only receive items if they beat the monsters AND
survive the entire battle. Consider changing gainItemSpoils to just keep
a list of what items were dropped.
3. Maybe an XNA thing, but when the player attacks and misses, the
monster also always misses. Perhaps the pseudo-random generator isn't as
random as we think.