fix initial location bug

fix threshold bug

remove stray dist file

fix destroy method (callmecavs#32)

optimize offset helper (uses getBoundingClientRect now)

update usage docs (callmecavs#40)

add container option (huge thanks to @wong2)

add gulp-header config

make CDN names headings

Merge pull request callmecavs#28 from sullinger/master

fix "element in viewport" calculation