Make default queue configurable and create one if none is provided

This makes the default queue used by timers configurable.  If no default queue is provided, create a new queue and set it as default.

This issue is blocking ember integration, because as it currently is, it uses the first queue in the array, which in Ember's case, is `sync`.

Using `sync` queue for timers defers observer firing because of `Ember.beginPropertyChanges` which is called before the `sync` queue is flushed.  

This causes issues which mainly show when setting a model property inside an `Em.run.next` or `Em.run.later`, the following jsbin displays the problem:  http://jsbin.com/ovefor/8/edit

Here's a fiddle showing how the problem is solved when a separate default queue is created (which is what my PR does): http://jsbin.com/ovefor/10/edit