Expose scroll behaviors, add test for ScrollToTopBehavior
Port ignoreScrollBehavior from #388 to the new API
[changed] Don't update scroll if only query has changed

Previously, the only way to opt out of scroll updates for a route would be by using `ignoreScrollBehavior`.
This, however, made it hard to implement arguably the most common use case: resetting scroll when `params` change and preserving it when only `query` changes.

This commit completely disables scroll updates when only `query` has changed.
This provides a reasonable default behavior and leaves `ignoreScrollBehavior` for more complicated cases.

If you'd rather keep the old behavior and reset scroll on query changes, you can either promote `query` variables to be route `params` or reset scroll position yourself in response to `query` changes in route handler's `componentDidUpdate`.

Fixes #432, #439
Merge pull request #522 from gaearon/reimplement-ignore-scroll

Add ignoreScrollBehavior to new API