updated extension to support nested matching pairs, remove inserted closing character on deletion of opening character of pair

introduced _matchStack to handle nested matching pairs and
_deletedToken to remember last deleted character.

Fixed issue where a closing matching character is not deleted when it
is exactly the same as the opening character e.g. ",',`
Editor now listens for 'keyEvent'

removed references to _lastCharacter and _document
set _enabled to true by default
Merge pull request #2 from thehogfather/master

Some enhancements to brackets-automatch-pairs extension