Don't use deprecated 'U' flag to read manifest

The universal newlines mode ('U' flag) is deprecated since Python
3.4. It only replaces "\r\n" with "\n", but it doesn't split lines at
"\r" (Mac newline). In practice, the flag was useless, the
sdist.read_manifest() method already uses line.strip() and so removes
newline characters.

readme: update links to documentation

Issue #593 and other fixes

Add a Gitter chat badge to README.rst

- Fix issue #593
- Fix some forgotten "vcver" => "vc_ver" renames in previous commit.
- Add comment on why I didn't followed PEP8 for some variables names.

Update link to Resource Management API