Event name added to event plus this.event available in events

Addresses #1249:

* `name` property added to `event` object
* `event` object accessible via `this.event` in handlers
* `event` created as `this.event` for `fire()` with `name`, `context` (`ractive.data`), `keypath` (`''`)

Still need look into deprecating default `event` as argument before this gets merged.