Add SlackEvent to data model

[rwblickhan]

Add a SlackEvent class to model/ to represent a Slack event parsed from JSON. In particular, it should have (de)serialization functions (to convert to and from JSON). It may also be helpful to have "sub data types;" for instance an enum for the possible event types (or at least the ones relevant to Rocket 2.0's MVP).

[rmcreyes]

It seems as though each Slack event's JSON string has an inner JSON object as a property, and this object has a different structure depending on the kind of event.

This is a full example of the structure of a Slack reaction_added event. Take notice of the event object:

The event documentation provides detail of the structure of other events, where in most cases only the event attribute's structure is shown, and in others the full structure is shown, like in this link_shared event:

In some cases, the event does not even have an event attribute at all, like in this app_rate_limited event:

The simplest way to handle this in my opinion is to serialize the outer attributes, as well as the type attribute of the inner event object to identify the kind of event at least, and leave the inner event object as a String attribute of the SlackEvent class being built, but I am open to suggestions as to how to handle this.

[rmcreyes]

Also since Python represents JSON objects as dictionaries with strings as keys, I also propose a solution that has the class expose the event fields through a function that takes a string and returns the value associated with the key:

where __event_obj is a dictionary.

[rwblickhan]

I believe we've decided not to go down this route, at least for the time being; closing.