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
Merge pull request #46 from qrator/master

Detect scroll to top/not top