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.
Please see https://www.npmjs.com/package/irc#help-it-keeps-crashing- for the faulting case. Otherwise please do this check so that we are conform with what eg chanserv does. It only allows actions for registered and identified nicknames.