feat: support for DISTINCT in aggregates other than COUNT

[wesm]

Issue by wesm Friday Apr 10, 2015 at 22:08 GMT Originally opened as http://github.mtv.cloudera.com/wesm/ibis/issues/179

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STDDEV, NDV, APPX_MEDIAN, and others support a STDDEV(DISTINCT foo)

[cpcloud]

There's a concrete use case here with array_agg and string_agg, but since no one is asking for it I'm going to close this out. We'll happily implement it if someone asks and/or someone puts up a PR!