Allow requirements file to specify PyPi repos other than the official one

Allow requirements file to specify PyPi repos other than the official one.

If you have an internal one, you can add
    
    --extra-index-url=http://your-pypi.com/simple
    
Or provide "extra-index-url" command line argument at runtime.

The compiled requirements.txt file will include this statement so installation
goes smoothly. This additional repo will also be used to resolve any dependencies.

This branch also includes https://github.com/nvie/pip-tools/pull/80 and https://github.com/nvie/pip-tools/pull/81