Move commands into a separate command module

For ease of maintainability, move all the mudpy.misc.command_*()
functions into their own module accessible via mudpy.command.*()
instead. In order to accomplish this, User objects now carry a
universe attribute so that commands they call can identify the
correct universe object in which to act.

Alphabetize the command functions, update the corresponding function
calls in the command elements, add the new module to the API
documentation, and include it in the dynamic module reload list.