Simplify scripts, format, allow for config-file readout, DRY

@pkorshunov, @amohammadi could you please review this and merge it. This MR includes the following changes:

• Format all help messages to 80 characters (POSIX compliance)
• Include examples in the help messages (moved from the README)
• Add instructions on the README on how to create an environment and how to setup python-gitlab
• Delay the import of gitlab till when it is actually needed, so at least the help message can be printed
• Implement import checks and print an clean error message if one of the packages (gitlab, docopt) is not installed
• Remove duplicates in the documentation/README and leverage the --help message from each script (DRY)
• Allow the scripts to work against ~/.python-gitlab.cfg or /etc/python-gitlab.cfg instead of requiring a token on the command line
• Ask for a token (via input()) if it cannot read the configuration file