Implement nextRotation

We'll leave the heavy lifting to getPieces. This is also the first
instance of a pervasive lazy initialization paradigm--we only calculate
each piece's rotation once (in getPieces) and then return this
precomputed result later.
Implement the private TetrisPiece(Point[]) constructor

Also fairly simple, since it just involves copying an array.
TPiece: Add width, height, and skirt fields

These will be returned by the accessors, which will compute their values
if necessary.