Fixed random spawning issue

Fixed issue where random starting spawn would place both starting tiles
in the same div. I stopped checking the cell's contents for '' (empty
string) and, instead, use the jQuery :empty selector (i.e.
$("div.cell:empty").

Also in this check in are methods to get all cells and get all non-empty
cells. They're unrelated to the bug fix but I wrote them before the fix
occurred to me so I'm lumping them in with this check in.