BUGFIX: Scrobble track only once when it plays multiple times in a row

Added new CHANGED state to process "metachanged" intents on track switching.
Probably the START state must support this behavior but I didn't want to break existing logic.

Fixes https://github.com/tgwizard/sls/issues/44 and https://github.com/tgwizard/sls/issues/99 issues.