Observable API and listeners fixups

[meejah]

This makes the ObservableMixin less fragile for use as a mix-in by eliminating its parameter-taking __init__ method, and some cleanups and docstrings. It also adds an (optional!) list of "valid events" to make users of .on() (or even internal uses of .fire()) complain if you mis-type an event name.

The other commit tackles the LTS listener API by fixing how they're fired/invoked. Doing the invokes in e.g. onJoin won't work with most of the code out there (including all the examples) because the API contract never specified to call super() (and basically no code does this). This means that the 'join' event is basically never fired properly.

So, as per LTS API discussions there are 5 events. Note that I made one subtle change that makes a big difference (to when 'join' fires):

• join: right before we call onJoin
• leave: right after we call onLeave
• ready: after all 'join' listeners have completed, and after onJoin has completed
• connect: right before onConnect
• disconnect: right after onDisconnect

Above "completed" means the method has run, and any future/deferred returned has also completed. The reason I made 'join' fire before onJoin has run and completed is, consider code like:

class Foo(ApplicationSession):
    def onJoin(self, details):
        yield self.call(...)
        # etc
        yield self.leave()

That is, things that use onJoin as essentially a "main" function -- do all their work, and then disconnect the session. In the above, if we wait for onJoin to complete before we fire the 'join' listeners, then we very counter-intuitively would fire 'leave' before firing 'join'.

So, now 'join' fires as soon as we get the Hello message, and then the previously-discussed 'ready' fires after onJoin has completed (and all 'join' listeners have completed). Also, this is implemented such that it doesn't matter which of the over-ridable methods in ApplicationSession you choose to override, and doesn't care if you call super() or not. Also, any errors from listeners are logged (but ignored).

[oberstet]

I like that! It brings a lot of non-invasive flexibility ..