Adds support for on="eventType" attribute in linkTo helper

Syntax of the `on` attribute is the same as in the `{{action}}` helper.

This is particularly useful on mobile when one wants to avoid the 300ms click delay using some sort of custom "tap" event. A global event type override can now be achieved like so:

```
Ember.LinkView.reopen({
  eventName: 'tap'
});
```