Basic $project support. Various operators.

Several improvements to aggregations.

Release 3.5.0: $min and $max operations.

Release 3.4.0: $currentDate support

Release 3.3.0: Dot-notation for upsert. write_options args

Release 3.2.1: Fix unexpected exception when $regex meets non-string

Release 3.2.0: Aggregation ($group) improvements.

Release 3.1.1: Fix $unset and find projection for nested fields.

Release 3.1.0: More operators. Code quality improvements.

Release 3.0.0: Breaking version to support pymongo 3.x