Router delegation

[bradgessler]

A parent router should be able to delegate message routing to another router. When delegation occurs, the parent router needs to set a reference on the child router so that we can perform loop detection.

[thoughtless]

It is now possible to do router delegation (with version 0.1.1) with something like mount MyOtherRouter.new. However, a reference to the parent router is not automatically passed through, and there is no support (yet) for loop detection.