Added option to customise the IronLocation anchor selector 

Not necessarily a perfect PR, but I just wanted to float the idea of being able to customise the anchor selector for Iron Location.

Binding to `a[href]` on window doesn't suit every application - and interferes with other libraries like bootstrap tabs.

For example, I've customised my selector to `a[href][data-router="true"]`.

Another fix would be to ensure that the IronLocation handler / selector is always last in the list of event handlers- this would allow custom event handlers to call `e.preventDefault()` alleviating the need for this change.