ReferenceProperty support

[susanlinsfu]

Reading the docs about Google Datastore at https://cloud.google.com/appengine/articles/modeling. It mentions properties which would be a great feature as many models require one to many relationships.

[jgeewax]

I believe you can do this by setting the property as a Key object.

We won't automatically de-reference this for you (ie, when you access the property it will return the key which you then need to get), but it's definitely possible to do...

[jgeewax]

@stephenplusplus , what are your thoughts here? It kind of gets us into ORM world if we start de-ref'ing keys for people automatically, right?

[stephenplusplus]

Yeah, I think what is wanted here is an ORM. We also have a very old issue that requested showing an integration: #87 -- is it worth looking into that again?

@susanlinsfu in case this is helpful, I made a schema validator for gcloud-node + Datastore https://github.com/stephenplusplus/gcloud-datastore-schema -- I'm still working on adding nested array validation, but feel free to take a look.

[stephenplusplus]

Feel free to continue the ORM discussion in #87.