Fixes to Mongo Mapper's select function for group operations:

1) Corrected mongo group parameters (finalize)
2) Store just the 'retval' value from mongo group in temp collection
3) Drop temp table after reading results instead of before
Made Mongo Mapper select fixes safe for php 5.3
Merge pull request #423 from eschultz/dev

Fixes to Mongo Mapper's select function for group operations: