use throttle instead of debounce

[missinglink]

this PR is a replacement for #16 which is preferable since it's using a well tested implementation.

It replaces _.debounce with _.throttle and updates the API. This is a breaking change.

[missinglink]

Actually.. I think we should change the settings to { leading: true, trailing: true }.

The rationale is that connection latency will only ever affect the first 'paint' of the UI.

If the browser is sending requests every delay D and the connection has latency L, then the UI 'paints' will always follow the same pattern regardless of the connection speed:

note: I'm just showing the 'deltas' ie. the difference in time from when the previous response was received

{ leading: false, trailing: true }

D+L, D, D, D, D....

{ leading: true, trailing: true }

L, D, D, D, D....

In both of these scenarios, L only appears once, but with the second config slower connections are receiving their first request as quickly as they possibly can.