Added option to scale pitch with transaction size

I've seen the suggestion thrown around on reddit, so I took the time to implement it.

There now is a checkbox to enable that option, which will make bigger transaction sound deeper. It'll use a logarithmic scale from 0.01 BTC to 1000 BTC, then it will cap to the deepest sound available. To prevent all small transaction from making the same, high-pitched noise, I added some fuzzing so that the sound played is ± 2 from the actual pitch corresponding to the transaction size.

Thank you for making this site! It's a nice background for me to listen to while working. =)