Option layout

Improved option layout (http://code.google.com/p/daisy-pipeline/issues/detail?id=235&q=owner%3Acapitan.cambio%40gmail.com&colspec=ID%20Type%20Status%20Priority%20Milestone%20Owner%20Summary%20Component%20Module)

The output is more or less:
```
19:09 javi at disorder:~/daisy/pipeline-cli $ ./dp2 help dtbook-to-zedai
Usage: dp2 dtbook-to-zedai [options]
        --i-source input1,input2,input3
                                     One or more DTBook files to be transformed. In the case of multiple files, a merge will be performed. (application/x-dtbook+xml)
        --x-output-dir  anyDirURI     (required) The directory to store the generated files in.
        --x-mods-filename  [string]   (optional) Filename for the generated MODS file
        --x-lang  [string]            (optional) Language code of the input document.
        --x-css-filename  [string]    (optional) Filename for the generated CSS file
        --x-assert-valid  [boolean]   (optional) Whether to stop processing and raise an error on validation issues.
        --x-zedai-filename  [string]  (optional) Filename for the generated ZedAI file
    -n, --name [NAME]                Job's nice name
    -b, --background                 Runs the job in the background (will be persistent)
    -p, --persistent                 Forces to keep the job data in the server
    -q, --quiet                      Doesn't show the job messages
```