Navigation: Implementation of navigate event must properly attach to/detach from underlying events

[gabrielschulhof]

Currently the navigate event attaches to popstate/hashchange when the first call to setup() is made, and sets a flag indicating that it has attached. Upon subsequent calls to setup() it merely checks the flag to make sure it has already attached.

We need to change this to attach a handler upon setup() and detach that same handler - identified exactly by means of its function pointer upon teardown().

[apsdehal]

Closing as fixed on 1.5-dev.