`iD.behavior.drag` is like `d3.behavior.drag`, with the following differences:
* The `origin` function is expected to return an [x, y] tuple rather than an
{x, y} object.
* The events are `start`, `move`, and `end`.
(https://github.com/mbostock/d3/issues/563)
* The `start` event is not dispatched until the first cursor movement occurs.
(https://github.com/mbostock/d3/pull/368)
* The `move` event has a `loc` and `dxdy` [x, y] tuple properties rather
than `x`, `y`, `dx`, and `dy` properties.
* The `end` event is not dispatched if no movement occurs.
* An `off` function is available that unbinds the drag's internal event handlers.
* Delegation is supported via the `delegate` function.
Note that this requires wrapping all of iD, but not
third party libraries, in a function and enabling
strict mode only within that function. We can't be sure
that our dependencies are strict-safe (d3 isn't).
