Add the ability to reload async child nodes

[grapoza]

Spawned from the conversation in #278

It would be useful to have the ability to easily reload the children of a node of those children were loaded through an async callback. For example, if a list of items was loaded from a server and a client only knew that something changed but did not have enough information to add a specific new node then a full reload would be required.

The behavior would be similar to the following manual process:

1. set the node's expanded property to false
2. set the children list to [] (probably not necessary, but why not clean up after ourselves)
3. set the internal model.treeNodeSpec._.state.areChildrenLoaded property to false
4. set the node's expanded property to true again to re-trigger the async load (after Change expanding behavior to trigger from a watch on the expanded attribute #280)

This could be exposed through a function and should probably honor the parent node's current expanded state (though of course the state of children would be lost, since they're getting reloaded).