DATAMONGO-788 - Projection operations do not render synthetic fields properly.

Introduced boolean isSynthetic() attribute to FieldReference to determine if the given reference points to a synthetic field, as this controls whether we render a simple include (1) or a concrete reference ($fieldName) E.g. isSynthetic() would be true for a field reference to _id.
DATAMONGO-788 - Polishing.

Slightly changed the way the the simple reference rendering for projections is implemented. Introduced an isAliased() method on Field to be able to determine whether the field reference has been renamed explicitly.

Original pull request: #90.