Read cell comments in .xlsx files

Works for the single file I've tried it on, for what that's worth. Could
do with a bit more testing.

Fixes python-excel/xlrd#37
Merge remote-tracking branch 'gthb/feature-cell-comments-in-xlsx' into feature-cell-comments-in-xlsx
Read .xlsx comments from Excel 2007 / GDocs

This has test cases for .xlsx files generated by both Excel 2007 and
Google Docs, and works for 't' elements nested directly under 'text' or
with an intervening 'r' element.

It also copes with the (common) Excel case where there are multiple 't'
elements: this occurs when a comment has an emboldened author name
Avoid use of XPath features absent in 2.6's ET
Merge branch 'master' into feature-cell-comments-in-xlsx
Interpret xlsx comments with no text correctly
Merge pull request #58 from stephenlewis/feature-cell-comments-in-xlsx

Read cell comments in .xlsx files