get excluded packages from a requirements file

[avoine]

It could be usefull to have an option like: --exclude-requirements FILE

[flupke]

I implemented it in 5285a10, could you test it before I make a release? (it's on the develop branch)

[flupke]

(and a typo fix in 667ddc2)

[avoine]

In PR #3 I've made some changes to the way
exclusions are checked because right now it could exclude more that one would like.
I also made the compassion case-insensitive.

[flupke]

We should use pip to parse the requirements file correctly: https://github.com/pypa/pip/blob/develop/pip/req/req_file.py

This would also address the issue in #3