Added various command-line options.
Created a foundation for adding more options.
Currently implemented: --port, --host and --onlinemode

Some server hosters, especially those who have payment options on a per-slot basis, customize runtime parameters to restrict server owners from exceeding their limits. Glowstone currently has limited support for command line options.

This commit adds a foundation for command-line options and implements --host, --port and --onlinemode

This commit also formats the help message that it shown when running GlowStone with the --help switch.
Fixed gradle manifest reference. Thanks @Grinch, @scooterboo, @lukespragg.
Changed wgen to treat 128 as world height in all cases (fixes #151).