Aggregate extensibility

To close ISSUE-15 it looks to me like we probably should have a  section "Aggregator  extensibility" analogous 
to the existing "Operator Extensibility" section in the doc. Just a quick list of what aggregate extensibility 
seems to need to consider: 

Any new agregate should probably handle the following questions: 

   * agg-uri(*)  how/whether the aggregate treats rows? 
   * agg-uri(?X) i) how/whether to treat errors ii) how/whether to treat unbounds?
   * Would we also allow aggregates with arityy different to 1 (like common in functions) e.g. agg-uri(?X ?Y) 
   * Is it necessary to define agg-uri( DISTINCT * ), agg-uri( DISTINCT parameters ) separately, or do we asume this implicitly for ALL built-ins?

Axel