Added support for exporting to Microsoft Office Excel file format

This feature adds a dependency to my other extension PHPExcel
(which is currently awaiting repository creation, see Extension:PHPExcel
for more information)

This feature was developed as to enable importing to excel without loosing
data integrity, as one would by using CVS export functionality.

If you tried to import from cvs a cell that contained the data "0320" excel
would auto interpet this as a number instead of a string effectively setting
the cell value to "320". One could workaround this by setting each columns
datatype in the import screen of excel, however this leaves that
responsibility to the user which is bad.

However using SMW data type information and PHPExcel library, it is possible
to explicitly set each cells data type. As of now 3 datatypes has been
defined.

Quantity: Displaying the data as a number and formatting the cell with the
specified unit type. (e.g the data "100 tons" will be set as 100
and displayed as "100 tons")

Number: Setting the data as a number

String (everything else): Displaying the data as a string
(e.g "0320" would actually be "0320")

Example usage:
{{#ask: [[Category:...]]
| ?var1
| ?var2
| ?var3
| format=excel
}}

In the case where a user tries to export as excel and the extension PHPexcel
is not installed on the system. The user will be prompted with a error message
detailing the missing extension.

Change-Id: I742699fecc0238c030cba20c7fd4bad5526e33ac
Merge "Added support for exporting to Microsoft Office Excel file format"