Support for functions on sub-docs

[davefinster]

Keen for some feedback on some small additions I made that allow for the attachment of functions to sub-docs. I was porting a codebase from MongoDB to RethinkDB and this functionality was required.

Sub-doc functions can be defined by simply adding then as a member of the object that would otherwise define the schema (as shown in the tests). When a document is instantiated, the '_overrideProperties' method is invoked to define getters/setters for any sub-doc properties such that the prototype can be injected. It does something similar for arrays of sub-docs by hijacking the push method on the attached array.

To support this, I've had to add the '_shadowObject' property to the document, which does mess with validation (which I've accounted for by adding exceptions in validation and ensuring it doesn't make it into the saveableCopy) and ideally I'd like to be able to hide it - but not sure what the best way of doing that is.