Added ability to mark methods as only usable by players

A little bit of code to make it easier for programmers to mark methods so that they are only usable by players. For example a teleport method would not make sense to be used by command line so a developer would likely want to mark the method with this.
Example:
```java
	@CommandHandler
	(
			command = "teleport", 
			permission = "essentials.teleport", 
			noPermission = "You do not have permission to teleport",
		    usage = "/teleport <person to teleport to>",
		    playerOnly = true
	)
	public void onCommandUpgrade(CommandInfo info) 
	{
            //teleport player here
            //this code is only reachable if a player calls the command
            //and not if someone on command line calls it
		
	}