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 just wasn't sure about indexing a table with a nil, I remember reading something about it but not the exact details.

It should work independently on platform, core Lua language is the same everywhere :)