Feature/mongo 2.6.5 accumulators MinMax

Update MinMax accumulator and tests for 2.6.5.

Combined Min and Max tests into one file to be more consistent with our other tests.

Here are the original source code files for reference:
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/db/pipeline/accumulator_min_max.cpp
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/dbtests/accumulatortests.cpp#L422

NOTE: this is dependent upon the updated Value class in pull request #4