Allow setting a meaningful value to event's @source field

Currently there are no way to set an arbitrary value (in theory, a URL) to the @source field, that was always set to 'unknown' previously.

This way, we can easily set a value that can be use later on to differentiate log messages coming from different sources (different apps for example).

Also, I added a small rake task to fire up a console with the LogStasher code already loaded, which can prove useful for hacking/debugging.