Increase accuracy on faster songs

Do a few things to increase the accuracy:
* Save 32 data points (vs 9)
* Allow the use of the keyboard, too, which
  causes less noise in the data than mouse

Also make the bpm calculation less obtuse.
Refactor: Use toFixed(2) instead of custom function

There's a function built into JavaScript for
cutting a float down to a fixed number of
decimal places, so let's use that.