Remove AbstractEvent and AbstractVenue

Been wanting to do this for while. The `SourceParser` subclasses were creating an `AbstractEvent` and an `AbstractVenue`, but then immediately invoking complex methods to convert them to a normal `Event` and `Venue`. The abstract versions were transient, and never used outside of this process, and thus unnecessary. This refactoring removes all that complexity, and now `SourceParser` subclasses create `Event`s and `Venue`s directly.

This exposes more low-hanging refactoring fruit among the `SourceParser` subclasses, but I'll save that for later.
Hmm, very procedural. What about a more functional approach?

```ruby
matched_parsers.collect { |parser|
  parser.to_events(opts)
}.detect(&:present?)
```

Thoughts?