New PlayerList Command (/present playerlist)

HelpList.java:
- added the new "/present playerlist" command to the staff and op help
list.

Main.java:
- added getCommand("present").setExecutor(new PlayerList());

PlayerList.java:
- added a new command "/present playerlist" for staff to list players on
the 'names' list in the config. This is so when they work on the
presents they can see who has asked for one.

plugin.yml:
- added the new command, the usage and description for "/present
playerlist"