Update README.md

Change URL to new format
Added Command.NumberOfArguments message
AbstractExecutor: Added some basic checking mechanisms
Removed reloading functionality from the /customjoin command
Make use of the AbstractExecutor checking facilities
Made the command checking system exception-based

Also, enhanced formatting and spelling here and there.
Use ValidationException instead of CommandException
Moved getMessage to the plugin class for global availability
Added the formatInfo utility method
Added CustomJoinPlugin member
Pass arround the player name instead of the actual object

... seriously, what was I thinking >_<
Added message previews and enhanced code readability
Fixed missing initializations
Removed unused import
Added chat formatting resets, previews are now accurate
Renamed the SetMsgExecutor to AddMessageExecutor
Added a getter method for the MessageFormatter
Refractored and changed functionality according to new name
SetPlayerMsgExecutor -> AddPlayerMessageExecutor
Renamed PlayerMessage to CustomMessage for name clarity