Untangle key purpose/substance (API breaking)

In practice, if you're using the Key.{continuous, discrete, meta}
or their magic equivalents, your code should work as usual.

Orange actually distinguishes between two characteristics in its
variables (naming mine):

- purpose: meta/class/feature
- substance: continuous/discrete (called 'type' in Orange)

It appears that in the CSV format the two are conflated and that
Orange tries to deduce the substance when unknown (not sure about
this; in any case, we don't specify what types the meta/class
features are)
Support tab-formatted Orange output
FIX always keep case for relation labels
stac-util insert: Support inserting into mid-text
Merge pull request #10 from moreymat/always-keep-case

FIX always keep case for relation labels
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