Allow multiple "req" lines per item
Change syntax for item requirements

Instead of "req=p,2", we now use "requires_stat=physical,2". Other stats
include: "mental", "offense", and "defense".
Merge pull request #1243 from dorkster/item_req

Item requirement improvements
Don't overwrite the character class with a generated title
Support unique power trees for classes
Don't display duplicate class titles
Add "requires_class" property to items
Better engine/classes.txt parsing

Also, don't translate "Adventurer", since class names are stored
untranslated.