Version 0.1.88

* Impress sessions refactored
* client.session - > client.sid
* client.application.sessions[client.session] -> client.session
* Refactored impress.security.js (according to session changes)
* Fixed examples (to comply new session changes)
* Refactored .register() .signIn(), .signOut()
* Moved sessionCreated and sessionModified to Client class
* Session cookie secret, close issue #89
* Warning: you need refactor code everywhere you use client.session