Ignore close callback when closed.

This fixes a bug where closing a bullet
connection in poll mode will not work.
Fix jshint warning.
Drop reference to xhr request when done.
Add buttons to demonstrate sending for all bullet connections.
Use xhr for sending with eventsource connections.
Merge branch 'eventsource-xhr-send' of git://github.com/jdavisp3/bullet
basic jsonp support, useful for cross-domain without websockets