[jopendocument] Support in-memory sheets (and documents read from an URL)

[Cafebabe]

The SpreadSheet-API is very File-oriented. I would like to see a way
of reading SpreadSheets from an URL or from an in memory byte array:

SpreadSheet createFromFile = SpreadSheet.createFromUrl( new URL
(url) );

and

byte[] bytes = ...
SpreadSheet createFromFile = SpreadSheet.createFromBytes( bytes );

Or in general an InputStream. This is much more flexible than a File.

SpreadSheet.create( inputStream );

Instead of saveAs() to a File I would like to see an OutputStream

OutputStream  baos = new ByteArrayOutputStream();
sheet.getSpreadSheet().saveAs( baos );