Add an option to prevent notifications autohiding

By disabling this option, notifications will not automatically close themselves after a predefined period has elapsed. By remaining open, notifications will remain in the Notification Center on OS X Mountain Lion, providing a log of play history.
Merge branch 'notification-center' of git://github.com/hayfield/Chrome-Last.fm-Scrobbler into issue-178

Conflicts:
	scrobbler.js