Feature/mongo 2.6.5 accumulators Sum

Updated Sum accumulator and tests from 2.6.5 source code. This includes a large number of tests that had previously been omitted.

Here are the original source code files for reference:
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/db/pipeline/accumulator_sum.cpp
* https://github.com/mongodb/mongo/blob/r2.6.5/src/mongo/dbtests/accumulatortests.cpp#L556

We don't _have_ to because it isn't ambiguous but we probably **should** for consistency.
Fixing.