Fleshed out Item and ItemDB classes

Fleshed out both the Item and the ItemDB classes. Some changes
were made from the design document. Now, ItemDB will contain four
items: Red, Blue, Green, and Yellow gems. Each of these games has
a name, a multi-line, unparsed description with replacable text,
a buying price, and an array of two-element arrays. This final
array contains the effects for the gem when equipped in the
corresponding slot (in the order Head, Chest, Arms, Legs). Each
element in this array is an array with two elements: the first
is the number to multiply the rank of the gem by to get the
stat boost, and the second is a constant representing the stat
that is affected when the gem is equipped in that slot.

Item has also been overhauled. It now takes a database index and
a rank, and pulls the corresponding gem. It saves all the
necessary info, and also parses the description of the gem based
on that gem's rank and modifies its name accordingly.