commands: remove rst markup from help strings (fixes #122)

commands: remove rst markup from help strings (fixes #122)

The command docstrings are used in two places:

* Command line help accessed by 'zpm help'. We output the docstring as
  raw text here. Processing the help would be possible: I implemented
  this for Mercurial, but the code is not packaged right now.

* Auto-generated documentation. Before version 0.1.5, sphinx-argparse
  only generated a table with the command line flags. In version
  0.1.5, it began including the docstring as well, but as an blob of
  raw text.

Since we now have two places where the reStructuredText markup is
shown directly, it seems better to remove the markup. The docstrings
are now a little boring, but suitable for display in the console and
as raw text online.