etherpad session use enhanced and SSL certificate verification configureable

* re-use session if its still valid for etherpad (session checked via cookie and etherpad api)
* verifySSL: true/false added to access pages with self-signed certificates
* documentation updated
* .gitignore updated to ignore dotfiles besides .gitignore
* tested with ruby 1.8.7, redmine 2.3.3, etherpad 1.3.0