Cleanup and update of `gamemode` commands

Updated gamemode console and in-game commands to behave in a more uniform manner, centralized into gm.lua to facilitate reuse of common code, updated Info.lua and README.md with additional information.
I did test it (both for the in-game and console command) and they work without issue on my test server - it returns a nil value which gets caught by the `if not Gamemode` check. Granted, the only server I have to test it on currently is linux based, but I'll see if I can grab a windows machine to test it and report back.