first pass at skeletal aggregation components
documents and fields to and from BSONObj
documents and fields to and from BSONObj
simple projections work
project array ravel working
FieldIterator to use BSONObjIterator pattern; field path expressions; initial comparison expression
rename ExpressionField to ExpressionFieldPath; build ExpressionCompare
nested expressions work
 works
Fields became immutable Values
create expression document, fix compile errors
minor edits
simple group-by working
add , ,
added  and  operators for aggregation
, , , virtual documents, refactored parser
accumulator efficiency, , and ValueIterator
file renaming and coalescing
run files through astyle
Merge branch 'master' into aggregation