Implemented basic persistance layer on top of LevelUp.
Moved the wiring to the Server in the persistance itself.
Fixed spurious failure in the levelup tests.
Restoring a client's subscriptions after a disconnect.
Using a global nop instead of creating an empty function.
Added the support for uncleaned clients.
Added the TTL to the persistance.

The dependency to node-level-ttl is to the git repo, as it needs
a patch that is still not on NPM.
Updates node-level-ttl to the released 0.2.0.
Made the persistances work even if called as a Function.
Added basic persistance layer for redis.
Added support for pattern subscriptions in the persistance.

Still there a few todos, mainly due to the restoring of the
subscriptions across startup of the persistance engine.
Added restoration support of the Qlobber data structure for LevelUp.
Added synchronization support for Redis persistance.
Limiting unclean client handling to QoS 1 subscriptions.
Updated Ascoltatori to 0.7.0
Added MongoDB as an optional dependency.
Added mongodb in .travis.yml.
Implemented mongo persistance.
Made Mongo persistance spec faster.
Added 'before' to the .jshintrc.