Separate usage into usage/help/description

This commit implements a clear distinction between usage, help, and
description as in the argparse module. usage shows how the command
might be executed on the command line. help is a short description
for a command. description is a longer description for a command.

Before this commit, we didn't have a way to change description without
changing usage. This could be a nuisance for someone who wanted to see
the generate usage. For instance, if we ran `manage.py somecommand -h`,
we've seen something like this so far:

    usage: Some descriptions for the command.

    optional arguments:
      -h, --help            show this help message and exit
      ...

If we added usage manually to the docstring of the command, then the
output of `manage.py -h` would become ugly. The only way to get around
these problems is to use diffrent variables, namely help and description.