[fixed] Window scrolling

The router now remembers the last window scroll position at various
paths and automatically scrolls the window to match after transitions
complete unless preserveScrollPosition=true is used.

This commit also introduces a flux-style architecture to the high-level
transitionTo/replaceWith/goBack methods.

Fixes #189
Fixes #186
Merge pull request #249 from rackt/window-scrolling

[fixed] Window scrolling