updated changelog
add autofit column width support

http://libxl.com/spreadsheet.html#setCol
"Value -1 is used for autofit column widths"
New feature introduced in LibXL 3.6.0
Merge remote-tracking branch 'jan-e/master' into autowidth
Merge pull request #83 from Jan-E/master

add autofit column width support
Merge remote-tracking branch 'upstream/master' into autowidth
modified readme, added docs and test for ExcelSheet::setColWidth() autofit
changed readme