DAS aggregators need to show the record if possible

[vkuznet]

When using certain aggregators, e.g. max, min, I should be able to show the record itself rather then min/max value of asked field. For instance, if user type

find block | max(block.size)

I should not only show max block.size, but also a link to a record with this value.

[vkuznet]

valya: (In 38c865a) Return None as db instances when MongoDB is down; modified das aggregators to return record id for min/max functions; add avg/median functions to result object; modified result object to hold result and rec counter; add helper das function to associate with each aggreagators, e.g. das_min; fix #52

Signed-off-by: Valentin Kuznetsov vkuznet@gmail.com