Subclasses for different Event types

[banksJeremy]

Instead of having a single Event type, with a long conditional chain that decides what properties to add based on the type of event, we could have a bunch of subclasses of Event for different types, with a constructor that would take even JSON data and return an Event instance of the appropriate subtype (or just an instance of Event of the type is unknown).

@Event.register_type
class MessagePosted(Event):
    type_id = 1

    def __init__(self, data, wrapper=None):
        super(MessagePosted, self).__init__(data, wrapper)
        self.content = data['content']
        self.text_content = _utils.html_to_text(self.content)
        self.user_name = data['user_name']
        self.user_id = data['user_id']
        self.message_id = data['message_id']

from chatexchange import events

event = events.make(data)
if isinstance(event, events.MessagePosted):
    print "Got message:", event.text_content

    assert event.type_id == 1

Because of the number of classes this would add, it might be appropriate to put them in their own chatexchange.events module. But maybe not.

This would be particularly valuable if we ended up implementing an interface as discussed in #43, where different event types would not just have different static attributes but would have different methods as well.

[banksJeremy]

53b156f passed my Travis and your Travis. Merging into master.