MySQL and some general bugfixes

Added "/maintenance setmotd <1/2> " command to edit the maintenance mode motd via a command
The Bungee version of the plugin will now automatically create a MySQL table if it is enabled, so you won't have to manually create one anymore
Fixed the ServerListPlus motd only changing on using the command, but not on the server startup
Fixed an error with canceling the start-/endtimer in the Bungee version
Fixed an error with removing players from the whitelist in the Spigot version
Fixed some other minor bugs
Some internal code cleanup