Specify which fields to be retrieved from mongo

[tfredrich]

https://groups.google.com/forum/#!topic/restexpress/l1T6LK_fEjE

[tfredrich]

This possibly has broader implications, as it's much like "field projections" that clients can manipulate the properties that actually come back in the response. So... the MongoDB part of it is just the start...

[tfredrich]

See also #115