Fixing column autofit

* Determine character to be used instead of using '0'. Excel documentation
        reads - "Column width measured as the number of characters of the maximum
        digit width of the numbers 0, 1, 2, , 9 as rendered in the normal style's
        font."
* Do not add extra characters for determining width
* Determine width using getAdvance() instead of getBounds().getWidth()
* Add padding pixels

References:
http://msdn.microsoft.com/en-us/library/documentformat.openxml.spreadsheet.column(v=office.14).aspx
http://support.microsoft.com/kb/214123
http://docs.oracle.com/javase/7/docs/api/java/awt/font/TextLayout.html