RFC: Making *Locations full-fledged Action Creators

As discussed in #252, I would like to be able to differentiate between user- and browser-initiated events.
This allows us to avoid trying to restore scroll position when user clicks on links.

@mjackson and I thought that perhaps turning `*Location` objects into full-fledged action creators helps achieve that goal.
Current path state would move into `PathStore` which becomes less sophisticated.

In this pull request, I'd like to ask @mjackson and @rpflorence to comment on whether we proceed with this approach, seeing how I changed `HistoryLocation` and `PathStore`. (Scrolling is not implemented here, but this PR is pre-requisite for implementing proper scrolling anyway.)

This approach is more in line with Flux:

* All actions (whether user's or browser's) go through the same flow
* PathStore only keeps track of store instead of doing stuff

The downsides (?):

* Custom `Locations` now need to know about `LocationDispatcher`, `ActionTypes` and `PayloadSources`
* The action-creating parts of `HistoryLocation` will be duplicated in all `Locations`, they get more fat

I don't think these problems are that bad, but I'd like a few opinions before going ahead with this.
Thanks!