[database] move database code in common/
[sessions] can configure session store in configuration file
[sessions] use configured session store to store express' sessions
add sessions storing dependencies and repository to package.json
[database] fix redis' get on undefined id
[database] fix some ID problems with databases
[sessions] rename session storage collection
fix coding style in mongodb.coffee:58
Merge pull request #76 from Paulloz/sessions

Sessions through DBMS