Added style for CHI Extended Abstract Format

Added a style for the CHI Extended Abstract Format, based on acm-sigchi-proceedings.csl but modified to use square brackets for citation numbers.
Rename acm-sigchi-extabs to acm-sigchi-extabs.csl
Fixed problem in id and self-link.
Another fix to id and self-link.
Renamed style according to the guidelines.
Merge pull request #908 from jozilla/patch-1

Added style for CHI Extended Abstract Format