Fix listing issues for repos without issues

Fixes #79.
Allow partial parsing of arguments

This is a move towards implementing #69. Basically if setup_parser()
returns true, parse_known_args() will be used instead of the regular
parse_args() and any unknown argument will be placed in
args.unknown_args.
Allow commands to override the usage string

This is another move towards implementing #69. If a command class
defines the attribute `cmd_usage`, then it will be used as the usage
string for that particular command.
clone: Forward unknown options to `git clone`

This allows passing any arbitrary option to git clone when cloning
a GitHub repository.

Fixes #69.
Use GitHub media type v3

GitHub is changing the default media type used soon. The changes
compared to the current default (beta) are minimum and this tool
shouldn't be affected. Just to make sure we are using a fixed version
(which now is the latest), we explicitly ask for the
"application/vnd.github.v3+json" media type now.

For more details see:
https://developer.github.com/changes/2014-01-07-upcoming-change-to-default-media-type/