Add Piece class to the TetrisPrototype model

Commit introduces Piece class defining pieces and their methods. In future the piece is about to be rotated, hence Piece#anchor is introduced (top-left point occupied by the piece).
Moreover, to be able to paint a piece on a canvas with an offset, Piece#translated_points method is here to do the (not so) heavy lifting.
Welcome TetrisPrototype::Game, that ties it all together! :)

In the commit also Piece::SYMBOL were moved out from Canvas#paint_shape - move display code away from the data models.
Update of TetrisPrototype::Game

Also two examples are provided.