Cleans sessions in leader.py

Closes all open sessions in leader module.
Adds task to bugs file
Concurrency changes to database

Activates autocommit and deactivates autoflush to sessions.
This should allow higher concurrency on SQLite.

If things work, I should remove my poor tentative to activate autocommit by myself
Merge branch 'master' of https://github.com/jlengrand/twitterboard