Handle scroll to top

- 'headroom--top' class is added when browser is scrolled to top
- 'headroom--top' is removed when scrolling down
- Callbacks onTop and onUntop can be registered
- Detection of 'scroll to top' takes account of the offset option
dist files including top detection
notTop class added when not on top - for #41
force update on init to ensure correct state
dist files
Revert "Always unpin if scroll exceeds offset"

This reverts commit 895a426b2898db5150837b7132e3d5d1317ac954.
Merge branch 'scroll-top'

* scroll-top:
  dist files
  force update on init to ensure correct state
  notTop class added when not on top - for #41
  dist files including top detection
  Handle scroll to top