Very basic refactor of HTML/CSS and JS. Yes, I removed vendor prefixe…

…s from

css. We are using flexbox without prefixes. Don't think we need them for
border radius, etc. Still a lot to be done though.
We are now responding to 'keydown' event as opposed to 'keyup' which
lacked the classic tetris behaviour of running the tetris down if the
button is kept pressed.

TODO: There is a bug to be fixed. When a tetromino is rotated at the
edge of the grid it wraps to next row.