Use `pip` to parse `requirements.txt` files.

The format for requirements files defined by `pip` allows lines that are not version specifiers:

* Comments (preceeded by a hash)
* Numerous `pip install` options, including `find-links` and `index`

See <http://www.pip-installer.org/en/latest/reference/pip_install.html#requirements-file-format> for details.

This change does not completely make `pbr` and `pip` behave identically: in particular, I've made no effort to parse `find-links` options out to add to `dependency_links` for setuptools. However, it does allow non-trivial requirements files for `pip` to work reasonably well with `pbr`.

(No, I don't know where `pip.req` is documented. It's not on the pip website. However, this endpoint has been pretty stable; I suspect it's fine to call it, since `pbr` already depends on `pip`.)