DATAMONGO-1536 - Add aggregation operators for array, arithmetic, date and set operations.

We now support the following aggregation framework operators:

- setEquals, setIntersection, setUnion, setDifference, setIsSubset, anyElementTrue, allElementsTrue
- stdDevPop, stdDevSamp
- abs, ceil, exp, floor, ln, log, log10, pow, sqrt, trunc
- arrayElementAt, concatArrays, isArray
- literal
- dayOfYear, dayOfMonth, dayOfWeek, year, month, week, hour, minute, second, millisecond, dateToString

Original pull request: #418.

Author: christophstrobl