fix(karma) check if user exists before granting karma

[thgaskell]

Makes use of the robot.brain.userForName method to check if a user exists in slack before attempting to grant karma.

Note: It doesn't look like this will return application users (bruddahbot 😢).

I needed to revert some changes from the last commit because there were a few CoffeScript syntax errors.

Also added back the token expression type and renamed the hasAtSymbol to isMentioned on the token expression.

[bmatcuk]

In the past, people have granted karma to "things", not just other users... and sometimes people use real names instead of usernames. I'm not sure I like restricting to just users. Thoughts?