Excel worksheet name length and Excel merged cells issue

--Please see the first of the two commits in this pull request (895a452) for a more detailed explanation of the changes--
Microsoft Excel will display a non-descript error popup if worksheet names exceed 31 characters in length.  This issue took me a few hours to track down and I'd love to save other developers from the same time consuming issue.  This commit checks the length of each worksheet name when it is written in Worksheet.toXml() and if a console exists, writes a console.log message about the long worksheet name issue. 
There is also an issue opening files created with excel-builder.js in Excel with worksheets that contain merged cells.  I was not able to completely solve this issue, but was able to get Excel to at least open the file without loosing the data inside.  This was accomplished by reordering the write order of 'mergeCells' and 'drawing' xml elements in Worksheet.toXml(). 
Both of these issue affect Microsoft Excel 2007 and 2013, but do not affect OpenOffice 4.