Use pip.req.parse_requirements to parse requirements file

b/c it knows how to handle various options like --index-url,
--extra-index-url, etc.
Refactor so pip creates InstallRequirement instances for us

Extract logic into get_requirement_set to simplify process_requirements
Merge pull request #25 from msabramo/parse_requirements_with_pip

Use pip.req.parse_requirements to parse requirements file