help: add help command

This command will work similarly to 'hg help' or 'git help'.
help: handle command specific help
doc: add help command to command list
help: split command docstrings into summary and description

The first line of the docstring is the summary and it is used as the
help text for each sub-parser. This is shown in 'zpm -h'.

The other lines of the docstring is used as the description of the
subparser. This part is shown by 'zpm CMD -h'.
new: add summary line to docstring
Merge pull request #102 from mgeisler/improve-help

Improve command-line help