Change the way of stage script

[wateret]

As is:

• The script must have functions like on_deploy, end_condition, ...
• The engine calls the lua function by those keywords

To be:

• The script must have only one function main
• The script must register functions via game:set_on_deploy, game:end_condition, ...

With this way(registration of callbacks), benefits are:

• No need to match any function names between the engine and a script, other than main
• Easily support optional or multiple callbacks
  • For example, user-defined events can be none or many.

This is also a prerequisite for #61.