Add Shiro interceptor

it creates a request attribute (subject) that contains the true Subject. For more information about why don't use directly SecurityUtils.getSubject :
http://jfarcand.wordpress.com/2011/07/13/quick-tip-using-apache-shiro-with-your-atmospheres-websocketcomet-app/
https://groups.google.com/forum/?fromgroups=#!topic/atmosphere-framework/-wBK7DS93Z0