Added "total" test of db:aggregate()
Fixed a type in the first agregate test
Properly give the field that should be aggregated
Changes to db:aggregate()

- removed redundant order_by block. The given aggregate functions don't care about ordering anyways (and the function had no order_by argument)
- return an value type. All supported aggregate functions but MIN and MAX always return floating point values. MIN and MAX return the data type of the column.
Added test for db:aggregate() average function
Added tests for db:aggregate() "max" and "min" calculation
Added the test for db:average() count
other minor changes
Added tests for all db:aggregate() functions
the second version checks if a query argument works
Bug fixes for db:aggregate() functions with query
Fixed the query argument for db:aggregate function
Merge pull request #4 from keneanung/fixDbAggregate

Fix db aggregate