Fix regression: failure to rename ids
Support basket features (for sparse data)

See http://docs.orange.biolab.si/reference/rst/Orange.data.formats.html#baskets
Demonstrate use of basket features in RST_DT.

This should probably be reverted or turned into an actually
useful feature
Change basket demo so we only have one.

Orange complains if you use more than one basket feature at a time
Use .tab extension for RST feature gathering