Add user verification for admin commands

hi @whimboo !

here's a patch for issue #28. I've run some quick tests on it, seems ok.
this could perhaps still be tidied up some, but it is far better than the code it replaces in the first patch.
I've added some error-checking and tweaked the adminhelp array to support all this.
Currently we pass in only a single argument, but my point is that the user has to make use of the correct command definition. We might have to do extra checks in the future, but for now that length check should be enough.