Skript Structure parser

• Reworked parser and function validation system to properly support calling function before defining it
• Function calls are now always allowed before definitions of the functions - configuration option doesn't do anything or appear in new Skript installations
• Commands defined by scripts are now registered to Skript's namespace, not Bukkit's (you can use /skript:cmd as /cmd), thanks to @RoyCurtis
• Spigot 1.9.4 is now officially supported, and updating to it is recommended
• Fixed a bug with click events (clicking a sign)

Note that function parser is kinda experimental at this point. It might need some more bug fixes...