improved error msg during doc.save()

New documents must have an _id before saving.

closes #1595
removed namedscope

never worked right. never documented.

closes #679, #642, #455, #379
some refactoring of Query.js
more History updates
docs; multi-mongos support
docs; NOTE about db safe options
docs; aggregation
aggregation; geoNear -> near

consistency with Query builder