* wrapped creatures, levelers, and planeswalkers' additional stuff in a so-named element
 * made rules-text a mixed type instead of just a string (to support reminder text as an element)
 * made the artwork stuff subelements instead of attributes
 * tightened up the string types to be token, where appropriate
set up the 'inheritance' JAXB extension
added in a couple missing spaces in empty tags
changed Card/@name to @title
instead of <card> use <land>, <spell>, <creature>, and <planeswalker> in the descriptor, so the XML can have better auto-complete and validation from just the XSD.
renamed the CardName key to CardTitle to match the attribute name it's unique-ing
changed card-set/name to /title to match the card elements
made the title and key elements of card-set attributes instead (like the card elements)
made card rarity optional (common is the default)
made the title and key attributes of card-set required, which I forgot to do when I converted them from elements
added property annotations to correct the leveler and planeswalker ability collection names
 * split the type/subtype stuff out of CoreGroup because the roles are different for the different card types
 * changed the cards to extend BaseCardType instead of implement CardType so that they can have a unified interface, even though not all methods will necessarily work for every instance (UnsupportedOpEx when they don't).
sample descriptor with a card of each major type pulled from the archives
moved rarity and color-indicator to be attributes instead of subelements
moved watermark to an attribute
suffixed 'Layout' and 'LoyaltyAbility' with "Type" so they're consistent
moved casting-cost to an attribute
made type, subtype, and type-modifiers attributes instead of elements
suffixed 'Layout' and 'LoyaltyAbility' with "Type" so they're consistent