add Server-Sent Events protocol to db changes API. close #COUCHDB-986
add Server-Sent Events protocol to db changes API. close #COUCHDB-986

This patch add support for the new specification of w3c by adding a new
feed type named `eventsource`:

http://www.w3.org/TR/2009/WD-eventsource-20090423/

This patch is based on @indutny patch with edits.

apache/couchdb-futon,apache/couchdb-futon