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.
Check for duplicate extra index urls.

If there's multiple requirements files with same extra PyPi,
check if it has already been found somewhere else so it
doesn't get added to the compiled file more than once.
Correct format of extra-index-url parameter

Specifying the option in the requirements file with an equals '--extra-index-url=blah' works, but is not the documented format. The documented format actually is just a space between the option and value. The parser has been updated to accept both, but output the proper format.
Merge pull request #82 from beeftornado/beeftornado

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