Issue 86: Don't let too many Mario coin sounds play at once.

This change adds a 5 second delay before another Mario coin
can be played. This is for situations where some background
task can add many bookmarks all at once (like Sync), which
without this patch causes many coin sounds to play all at
once which is quite unpleasant.
Merge pull request #87 from mikeconley/coin-timeout

Issue 86: Don't let too many Mario coin sounds play at once.