DATAMONGO-769 - Improve support for arithmetic operators in AggregationFramework.

We now support the usage of field references in arithmetic projection operations.

Original pull request: #80.
DATAMONGO-776 - Improvement in TypeBasedAggregationOperationContext.

TypeBasedAggregationOperationContext's getReference(…) doesn't use a PropertyPath anymore to avoid running the camel case property resolution.