Simplified code to display hint pieces

Starting work on getting the game to finish, I realized that the code to
create and display a piece will make it easier to help finish the game.

Worked on #2, #3, #6

	modified:   blockus.js
	modified:   pieces.js
Possible moves for each player are now displayed

As part of the work to determine when the game finishes, now all the
possible available moves for each player are calculated. Still a work in
progress, as available moves are never removed if they become invalid.

Worked on #2, #3

	modified:   blockus.js