optimize imports

- load times were hitting 1+ second on rhel5 + python26 due
  to discover_commands() importing all modules too aggressively.
  This fix causes dbsake to only loads top level packages and pushes
  imports into the command functions to lazy load them on a per
  command basis
add docstring to mysql_sandbox command
tweak baker's formatting of command docstrings
cleanup whitespace in mysql-sandbox docstring