Check for Storage functionality

Check for Storage, and disable high scores if it's not present.
Check for SVG and classList support

Instead of assuming we have everything, check for SVG and classList
support before building the board.
Always add new high score if there's room

Originally, only two conditions were checked before adding a new high
score: was this list empty, or was the current score better than the
worst of the old scores. This missed one obvious case: is there any room
on the high score list, regardless of how good the current score is.