UX Tweaks

Two little updates for some things that were nagging me:
* Previously, if the user scrolled slowly (below tolerance), it was easy to scroll to top without re-pinning (just scroll slowly with a larger offset to see). This additionally checks to see if user goes below offset, regardless of tolerance.
* For shallow pages, it was easy to trigger pinned status by "bouncing" against the bottom of the page (Mac Chrome/Safari). This adds to the same check that prevents this issue at the top of the page.

These are not well-tested across browsers, but hopefully they're of some use.
Merge pull request #32 from davidkaneda/master

UX Tweaks