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
Ignoe anchors ($) in cell references
Revert cell_name_to_rowx_colx.  Remove anchors in do_row only.
github issue 64: need to skip meaningless chunk of 4 zero bytes between two otherwise-valid BIFF records; was being treated as a DIMENSIONS record and then crashing.
Don't use str.replace on cell_name
Merge pull request #65 from Bklyn/master

Ignore anchors ($) in cell references
Read xlsx merged cell elements.

Added option to read merged cells from xlsx files.
Merge branch 'master' into feature-cell-comments-in-xlsx
Interpret xlsx comments with no text correctly
Remove miscellaneous workarounds for pre-2.6 Python

* nested scopes enabled in 2.2
* bool() added in 2.3
* “int” is a type object since 2.2
Remove ZipFile.open() workaround for pre-2.6 Python
Use exception handler target variables rather than sys.exc_info()

In Python 3, saving the exception object manually will create a reference
cycle via the traceback attribute. Using the exception handler method means
the variable is automatically deleted, avoiding the reference cycle.
Fix variable name error introduced in commit 83b9643
Merge pull request #79 from vadmium/fix

Fix variable name error introduced in commit 83b9643
Merge pull request #80 from vadmium/py2.6

Remove workarounds for pre-2.6 Python
Merge pull request #67 from jmcnamara/merged_cells

Read xlsx merged cell elements.
Merge pull request #58 from stephenlewis/feature-cell-comments-in-xlsx

Read cell comments in .xlsx files