Parse XLSX files with unusual structure

[stenlarsson]

Normally you will find workbook.xml in the xl directory, but recently we got a file where it was located in the root.

To find the actual location of workbook, we must parse the file _rels/.rels. A separate rels file located relative to the workbook then points out the locations of the sheets, styles and shared strings.

[stenlarsson]

This has conflicts with master because I based it on 2.5.3 which doesn't have the performance regressions described in https://github.com/pythonicrubyist/creek/issues/122.