Remove legacy `hook.params.gist` `hook.params.hook`, etc

[Marak]

Since we resolved #166, we no longer need to pollute the hook.params scope with the new internal hook.resource scope.

Best we just remove these extra parameters from hook.params entirely ( since they aren't needed anymore and they are technically "unsafe" to use )

The only consideration we have is to make sure any existing hooks which use hook.params.gist, etc, don't break. I'll run a migration script and alert any users of the pending breaking change.

It shouldn't be terribly breaking change, since the previous scope for these was never quite safe to use anyway

Ref #217

[Marak]

Done: http://hook.io/examples/echo