Don't propagate the linkTo event if there is no href

For someone wanting to get an active class for an element other than a link, the currently recommended solution is the following:
http://stackoverflow.com/questions/14412073/assigning-active-class-to-selected-list-item-in-emberjs

However, it has a few problems, since the event will be bound to the non-link anyway.
I've fixed this with a [custom helper](http://dmathieu.com/articles/development/emberjs-twitter-bootstrap-active-li/) in my own applications.

This is a more clean and broad fix.
If `href=false` was provided (or the link isn't an `a`), we don't propagate any click event.