Start at Bottom or Page when Scrolling Back

When returning to a page by scrolling up, make sure we end up at the
bottom of the page. This is more intuitive than starting at the top of
the page if pages are longer.
Allow Inverting Scrolling Indicator on Page Basis

Allow page types to set an `invertIndicator` attribute on the page
element to control whether the scrolling indicator should be
inverted. Determined by presence of `invert` css class on page element
by default.
fixes editor preview panel scrolling/height issue
enables highlighting active chapter in progress bar
Additions to the Interface of the Scroller Widget

* Make scroller position accessible
* `onScrollEnd` and `onScroll` method
Merge pull request #177 from tf/highlight-chapter

Enables Highlighting the Active Chapter in Progress Bars
Merge pull request #178 from tf/scroll-back-bottom

Start at Bottom or Page when Scrolling Back
Merge pull request #180 from tf/invert-indicator

Allow Inverting Scrolling Indicator on Page Basis
Merge pull request #181 from tf/scroller-additions

Additions to the Interface of the Scroller Widget
Merge pull request #182 from tf/editor_previewcontainer_overflow_fix

Fix Editor Preview Panel Scrolling/Height Issue