Snail Mail

Added a permission and changed commands format a little. Permission mail.admin allows a player to use the /sendmail command. Mail can still be sent via mailbox without a permission. Command /sendmail now uses true/false instead of 0/1 to specify whether the virtual mail should be sent anonymously or should display the sender's username. The help message sent if the /sendmail command is entered incorrectly is now grey instead of white. Custom colors will come later.