Events: on('foo') -> onFoo()

[tjvr]

Part of #25.

What?

• onFoo instead of on('foo').
• onNextFoo instead of once('foo').
• (emitFoo instead of emit('foo') (nb. only used internally anyway.))

Why?

• onTap(cb), onDrag(cb), onDrop(cb) is more consistent with forever(cb).
• Shorter, less typing, friendlier to beginners.

[tjvr]

@PullJosh Do you agree? :-)

[PullJosh]

Making lots of separate, similar functions feels a little weird, but it's probably the best (simplest) option for users.