adding Operations to GroupByScan
* >as< to specify the name of the resulting aggregation column
* >min< and >max< as new operators
* >distinct< option for count operator (naive distinct algorithm)
* unit test for empty tables
Merge pull request #152 from lordgeorg/groupby_ops

adding Operations to GroupByScan
* >as< to specify the name of the resulting aggregation column
* >min< and >max< as new operators
* >distinct< option for count operator (naive distinct algorithm)
* unit test for empty tables