Impress sessions refactored

* client.session - > client.sid
* client.application.sessions[client.session] -> client.session
Examples fixed
Refactored impress.security

* Refactored .register() .signIn(), .signOut()
* sessionCreated and sessionModified moved to Client class from session
hash
* Examples fixed
Merge master to session
Session cookie secret, close issue #89
Merge pull request #101 from tshemsedinov/master

Changes from master branch
Merge pull request #108 from tshemsedinov/master

Changes from master branch
Merge remote-tracking branch 'origin/master' into session

Conflicts: lib/impress.sse.js
Merge branch 'session' of https://github.com/tshemsedinov/impress into session
Merge pull request #110 from tshemsedinov/session

Session branch