Allow filtering by events' type

Per #85, I have also [explained](https://github.com/cvut/sirius/issues/85#issuecomment-67648254) the reason to use `event_type` param.
It may _look_ better, but it _is not_ better, there is too magic and it leads you to a dead end. For example, you can't use instance variables inside of the block precisely because it is what `instance_eval` causes. E.g. this:

```
.then_if(@type) { where(event_type: @type) }
```

…calls: `where(event_type: nil)`. It _looks_ right but it doesn't work. And you don't have chance to understand it without digging deep into `then`'s source.
Screw aesthetics, think a bit about risks of shooting yourself in a leg, alright?