Fix python setup.py --help-commands not working

[althonos]

With the current green version installed:

python setup.py --help-commands
usage: green [options] [target [target2 ...]]
green: error: unrecognized arguments: --help-commands

This is a mistake of mine, as generating the argument list for green.command.green actually parses the CLI arguments beforehand, and since it does not know --help-commands, the program crashes.

I also changed the description to be more in par with the other tools:

• behave description: Run feature tests with behave
• nosetests description: Run unit tests using nosetests

so I changed green description to Run unit tests using green.

[CleanCut]

Thanks!

[althonos]

@CleanCut: I didn't even see there was an issue about this in the issue tracker ! :)

[CleanCut]

No way! I thought you were addressing that specifically because you saw it!

[CleanCut]

Included in 2.12.1 -- just released.