Make push()/update() navigate, and change state interface

Closes #36 by moving from appHistoryEntry.state to getState() and setState() methods. This adds the ability to modify the state of non-current entries as a bonus.

Solves much of #42 by making push() and update() navigate by default, although it does not yet include the initiator idea.

As part of this we simplify update() significantly, since now if you want to change state without performing a navigation, you would use setState(). This makes the URL argument required for it. For symmetry, I removed url from the options for push().