Restrict the module to only allowing access to the SlackClient% object.

Rather than return a list of voids, why not return the JSON->hashMap that gets processed and adds to the channels or users? Also, remove the default `#f` value from the `hash-ref` since a check as to whether the type exists is done in the `when`.

I'm sure there's bugs still lurking there and I still need to turn this into a proper module but I can make a call to "users.list" successfully so I'll commit this, now.

Initial commit