Space-separated events

This PR adds support for multiple space-separated events sharing the same callback, much as is already the case with `ractive.observe()`:

```js
ractive.on( 'foo bar baz', handleFooBarOrBaz );
```

The same also applies to `ractive.off()`. The order doesn't matter, and sloppy whitespace is handled:

```js
ractive.off( 'baz  foo ', handleFooBarOrBaz );
```