Allowing XLSXView to set openpyxl's read_only property as True or False

[mbelmadani]

Modifications to allow XLSXView to set read_only as True or False; previously, default was True and caused an issue with LibreOffice files where the maximum number of rows read was 65536. Resolves #437 .

Sample files to reproduce issue:
100krow-nofunc.xlsx - 100k rows, create with LibreOffice which causes only 65536 rows to be read in if read_only = True.
Book-byMSExcelOnline.xlsx - Created with Microsoft Excel and does not have the 65536 row limitation on read_only = True.

nosetest -v and tox -e py27,py36 all passed after modifications.

• Unit test coverage has not decreased (see Coveralls)

• Docstrings include notes for any changes to API or behaviour

• Travis CI passes (unit tests run under Linux)

• AppVeyor CI passes (unit tests run under Windows)

• All changes documented in docs/changes.rst

• Includes unit tests

• New functions have docstrings with examples that can be run with doctest

• New functions are included in API docs

[coveralls]

Coverage increased (+0.02%) to 91.241% when pulling 1601b0a on mbelmadani:master into 02b7934 on petl-developers:master.

[alimanfoo]

Going ahead with merge, happy to revisit if any comments.