Update LastFmDp schema (at, artist, name, album). Flesh out LastFmImporter
Move Util/mix_panel to Util/Log, add Util::Log.mixpanel, Util::Log.warning. Update rest of code to use this
Clean up LastFmDp / DataPoint hierarchy: move at field from lfmdp to DataPoing, remove 'started_at' field. Add DataPoint.latest. Update default scope to sort desc by 'at'. Update LastFmImporter to generate time range from previous LastFmDp's timestamp. Update scheduler to import_last_fm