Rename "tracker.js" to avoid it being picked up by adblockers

Hello,
Firstly let me thank you for the great work with Ackee. I was looking for simple, self-hosted analytics, and it works perfectly for my usecase.

I had a small problem with the script tracker.js, because of the name, it is picked up by adblocker filters. Maybe it makes sense to rename it to ackee.js or something along those lines.

Cheers!