Scrobblers correctly handles track switching.

Hi.
This is the humble reincarnation of pull request #100. It fixed #44 and #99 for the most of the players.
Also I couldn't reproduce an issue with Google Play Music that @bmos have mentioned, so either developers have fixed it or I just forgot the correct scenario for it.
This fix doesn't resolve problems for all of the players which seem to be impossible, but I hope it helps in most of the cases.
I've checked it in PowerAmp, Google Play Music and Player Pro on SGS III. It'll be very helpful If some of you could also try this fix on your devices/players.