Correct variance inequality

Now correctly tests variance when deciding if we need a new time
signature.
isRunning bugfix

Add missing ().
Extend onBeat callback's args for UI

We now include the current bar and target number of beats to save the
UI needing to keep this as stage / work out the details.
Add a simple UI

This is based on the demo page I'm building on my own website so it's
pretty simple for now.
Initial docs for UI