Fix xlsx sheet->comments relationship

Instead of the previous bogus assumption that `xl/commentsN.xml` always
referred to `xl/worksheets/sheetN.xml`, now find the correct comments
file for a given sheet via `xl/worksheets/_rels/sheetN.xml.rels`

Closes python-excel/xlrd#108

(This is a squashed version of https://github.com/python-excel/xlrd/pull/109 hoping to get Travis to try a fresh build.)