srf.formats.datatables() enable table content export (use of SMW/SRF export printers)

@version 0.2.5

+ Add exports links for { 'csv': 'CSV', 'rss': 'RSS', 'json': 'JSON', 'rdf': 'RDF' }
Links will change with the limit set in the query panel and will adjust
to a columns visibility which means content generation is adjustable
on-the-fly without the need to alter the in-page #ask query.

+ Use extended smw.dataItem
+ Fix max limit value display

## Depends on
+ I1957236bcd4
+ I67bd14327a9

Change-Id: I1c59d61db8a9c073692ad61003f0ed5efd22c35a