add -e/--env command line argunment

This command line argunment allows someone to pass an environement variable to
gunicorn:

	$ gunicorn --env FOO=1 test:app

With the command line above the application will be able to use the FOO
environment vatriable.

This command line allows people to set any environment variables without having to know how to pass them in their shell. Also it can be used as a way to set custom options in workers.