Add 'off' method to ListWatch and Informer which allows for the removal of callbacks

[benbuzbee]

Resolves #322

Why was a call to slice on each array added before passing to calling functions?

Without it, the second test added, "mutating handlers in a callback should not affect those which remain", will fail. The calling functions use Array.prototype.forEach. From MDN

If elements that are already visited are removed (e.g. using shift()) during the iteration, later elements will be skipped

Will slice be slow?
Slice makes a shallow copy of the array, its linear with the number of callbacks added for a particular verb. Overhead should be minimal. If it is a concern, a more complex solution could be devised where .on and .off do not write to the callbackCaches during a forEach, however I prefered simplicity.

Why was the initialization of the callbackCache verbs moved to the constructor
Without it, more complex semantics are needed to determine if it is safe to call .slice in the various locations (i.e. callbackCache[VERB] may be null and .slice would fail without a check).

I think this centralizes the location of verb checking now that it happens in two locations nicely

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[benbuzbee]

/assign @mbohlool

As asked to do so by bot

[brendandburns]

This looks great! Thanks for the detailed PR.

/lgtm
/approve

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