Store the marble color in a data attribute
Don't try to unbind events from removed marbles

If we're emptying out the color chooser in one line, it doesn't make
sense to try to find its children in the next line.
Separate building game from resetting board

Resetting the board is more of a UI function (so we'll move it to the
App.ui object soon).  Building a game is a separate function that
should be called independently.