Make command registration goroutine safe

We should add a mutex around all places that register commands. By doing so, it would be much easer to register functions dynamically.

For example:
!send-periodically "I like cats" "0 0 0 * * *"
Would then send, in the current channel, "I like cats" at midnight every day.

I am sure more interesting things will be considered by allowing dynamic registration of commands, and making the registration threadsafe is just the first step to enable that excellence.