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.
value can be None
remove useless lines in example
add --chdir option

chdir to specified directory before apps loading.

Ex:

	$ gunicorn --chdir ./examples test:app

fix #384
fix tests
monkeypatch wsgi.sendfile for gevent

make sendfile usage non blocking with gevent