Add set_calc_mode with tests and docs
Merge pull request #92 from renshawbay/calcMode

Add set_calc_mode() with tests and docs.
Fixed note for Python 3

io.StringIO will raise TypeError: string argument expected, got 'bytes'.
Use io.BytesIO instead.
Added http_server_py3 example.
Merge pull request #96 from krysros/master

Added http_server.py example for Python 3.
Renamed python2 example file.
Add option to add hyperlink to image via insert_image

Add option to add url and tooltuip via the options parameter.
Merge pull request #99 from tax/image_links

Add option to add hyperlink to image via insert_image().
Fix for insert_image() issue in 'in_memory' mode.

Fix for missing images in in_memory' mode. Issue #102.
Fix for '0' as a number format string.

Added fix to handle '0' and other number like strings as
number formats. Issue #103.
Merge pull request #2 from jmcnamara/master

Merge pull from jmcnamara/XlsxWriter