The Quassel Core now remembers on exit which networks where connected and which channels

The Quassel Core now remembers on exit which networks where connected and which channels
everybody has joined, and restores this information upon restart (so all networks will
automagically connected and the appropriate channels rejoined).
Has not been tested with more than one active CoreSessions.
You can start the core without session restore by using --norestore as parameter.
Closes BR #63.