Fix typo in intro.md

The typo "effecient" becomes "efficient".
Merge pull request #25 from johnbacon/patch-1

Fix typo in intro.md
Detect correct transitionend event name

Improves support for slightly older Opera, FF, and Webkit browsers
Actually use the detected event name
Merge pull request #29 from leereamsnyder/transitionend-names

Detect prefixed transitionend event names