Make event args readonly

[transitive-bullshit]

This continues the work in #17 to make Dexter's core more immutable and easier to reason about.

Specifically, any of the objects passed to event handlers should be readonly to guarantee that event handlers don't have unintended side effects for future event handlers or internal functionality.

We may want to consider making other key objects like AbstractModel.params deep readonly as well.

[transitive-bullshit]

Bumping this now that #17 is merged. IIRC this was the cause of a nasty bug in Dexa awhile back.

[rileytomasek]

shouldn't this be fixed by replacing all of the methods that mutate state with extend?

[transitive-bullshit]

Possibly; it's been awhile since I've looked at this code. Feel free to close if this is handled already.