added more export options + file extensions + credit + unsuccessful try of adding a command line

Added 3 export options for native Google docs, handled via a "export format" dictionary :
* export to MS Office formats (gdoc> docx, gspreasheet > xslx, gpresentation > pptx)
* export to Open Office formats (gdoc > odt, gspreadsheet > ods, gpresentation > pptx)
* export to pdf (gdoc > pdf, gspreadsheet > pdf, gpresentation > pdf)

It can be improved.

Didn't succeed in making these options available through a "export" command line.

I also added the handling of file extensions (native GDocs were exported to files without extension). I didn't notice side effects, but it can't hurt to investigate a little more.