Improved events

[barneygale]

Good things:

• Event handlers with bukkit-style priorities
• Handlers are processed sequentially (no concurrency, even with the same priority).
• Handlers return a deferred. We add a callback to run the next handler.
• .dispatch() returns a deferred, triggered by the final handler completing.
• Formalize 'requires' and 'contains' into one structure, with optional defaults.
• Rename 'consider' (and sometimes 'predicate') to 'filter' or 'prefilter' and try to improve/document its interface.
• The handler should decide whether to unregister itself, not the filter. Filter just returns boolean True/False for whether the handler should be called.
• Move action chain into a class with a less functional-style approach. The code is hard to follow and doesn't allow for checking the time til the impending restart, for example.

[edk0]

we should consider not reversing the priorities as bukkit does (highest is called last). high-priority event handlers should have a greater chance of seeing an event before it's eaten than low-priority ones.