Add support for EmbeddedEntity in Datastore

[volgin]

Currently there is no support for embedded entities and no mentions of them in the documentation or Datastore example.

[mziccard]

Actually this is supported.
You can set an entity as a property of another entity by using either set(String, Value<?>) and passing an EntityValue or by using the newly released set(String, FullEntity<?>).

[volgin]

It will be nice to add this information to the Datastore example and JavaDoc.

[mziccard]

It will be nice to add this information to the Datastore example and JavaDoc.

Javadoc should already be in place. I will take care of adding this to the example.

[mziccard]

An example for datastore's embedded entities has been added in #980, see here.