Add a way to calculate the mean of the values in a particular field
Add missing test for the new 'Mean' operator
Merge branch 'master' into custommapreduce
Add a test for the new 'Mean' query (it's not implemented yet, so it doesn't work)
First pass at a map/reduce implementation of Mean
Shouldn't count values for records that don't contain the field to average. The result is NaN if there are no values to average together