Use unix line-endings.

Many of the files in this repo had mixed DOS and Unix line endings:

```bash
# cat -e setup.py |tail -10
    zip_safe=False,^M$
    install_requires=["gevent>=0.13", "flask>=0.8", "requests>=1.2", "msgpack-python==0.3.0"],$
    tests_require=['unittest2', 'mock', 'pyzmq'],$
    entry_points={^M$
        'console_scripts': [^M$
            'locust = locust.main:main',^M$
        ]^M$
    },^M$
    test_suite='unittest2.collector',$
)^M
```

This standardizes the repo on Unix line-endings. Unfortunately, Github doesn't show line endings in the diffs, so the summaries here aren't helpful. 

There is also a .gitattributes file to help normalize the repo going forward using the suggestions from https://help.github.com/articles/dealing-with-line-endings.