Add --install-options and --global-options to requirement parser + refactor

Hi all,

This pull requests adds the functionality requested in #271 and #515. In brief, it allows you to pass `--install-options` and `--global-options` in the requirements file. Example:

```INITools==2.0 --global-options="--help" --install-options="--prefix=/opt"```

I've also completely refactored  `parse_requirements()` and added more tests.

Reviewing by commit might be easier than going to 'Files Changed', imho.