add support for square highlights / modifications

[oakmac]

Users should be able to highlight / unhighlight squares via the API.

• .clearSquareHighlights()
• .squares(), .getSquares() --> return data about the squares
• .setSquare("e2", "blue")

[ZenLearner]

Most of the functionality one would ever require is in the very nice examples (as noted a couple are giving errors)

I was going to add an Event Listener for adding Squares to highlight and then calling the very nice AddCircle Function.

Then upon seeing your wonderful implementation of adding arrows by clicking on the begin and end squares (Sorry dragging)

board.addArrow({
start: startingSquare,
end: evt.square
})

thought you may do an example on this... (First click on the squares to highlight and then doing an addCircle button to Highlight them...)