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.
Hi @whimboo 
We do already check that this is an admin command we know about at the top of the block, to avoid "Sorry! You're not an admin" messages in response to other PMs, including non-admin commands.
I've added a default block, in case we add an admin command to the admin help block without adding its code.