Map event and vector source exports

The `map` and `frameState` properties of `ol.MapEvent` are not currently exportable (though `map` is not renamed by chance).  I think we do need to revisit the `postrender` event.  Having `postrender` be a `ol.MapEventType` while `precompose`, `postcompose`, and `render` are `ol.render.EventType`s is bound to be a source of confusion (see [this comment](https://github.com/openlayers/ol3/pull/2063#issuecomment-43048431) for example).  I'll open up a separate issue for that.  But in the meantime, it would be useful to have these properties be exportable.

In addition, the `getFeatureById` method should have been made exportable in #2087.