Don't redeclare 'execute' method on DynamicCommand

The gen-class macro doesn't need to be given a ':method' clause
for any method that's already declared by the parent class, even
if the parent class (in this case, org.bukkit.command.Command)
only declares the method as an abstract signature.
Make 'rokkit.commands/register-command' public

Functions called directly by the 'defcommand' macro need to
be made publicly visible to ensure that 'defcommand' may be
called from within an arbitrary namespace.
Introduce 'rokkit.bukkit' namespace

This namespace will contain functions that deal with the
Server singleton. Currently it contains only one function:
a quick way to get a reference to the server.