Removed unused import.
Revised helper methods to be private static rather than private to theoretically reduce memory footprint for each Promise instance.
Fixed an issue where an error thrown by a scheduled callback stopped all future execution of scheduled callbacks.

* Revised `nextTick()` to perform safer next tick scheduling - i.e. where a scheduled callback can throw an Error without affecting other scheduled calls.
* Modified `log()` and `done()` to use this safer `nextTick()` implementation to rethrow errors.
* Moved the optimized CallbackQueue to be an internal class used only by Consequence, where its underlying assumption (that execute() will not throw an error) is valid.
* Cloned additional CallbackQueue performance optimizations from promise.coffee and Deft JS - ensuring that only a single Array instance is used, rather than allocating new Arrays (via push(), etc.) whenever new callbacks are added.

Fixes #29