Added chat functionallity making the bot able to send messages to the game chat for everyone to see. Used by calling self.send_chat(<message>).

Instead of having two ways of using abilities directly, have one
way which is consistent with the API. Now unit has one method called
`ability` which takes a AbilityID and optionally either a Position or
another Unit which to use the ability at.

fixed indents to match previously written code

simple python edit