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.
There's talk of it but no decision has been made. We could introduce [backports](https://github.com/marcandre/backports#ruby-20-backports) for `Enumerable#lazy` for `1.9` or revisit the failing spec.

Which spec fails?