Automating of subdocument's loading

This pull request just for discussion, what do you think about this feature? I found it very useful in many situations.

Example of usage can be found in comment to EMongoArrayModel class. I try to avoid  automatic loading of all classes so by default behavior of extension was not changed and attributes not mentioned in subDocuments function will be loaded as php arrays.

Also I need help with re-factoring of EMongoModel (now I have two almost identical functions in EMongoModel::getRawAttributes() and EMongoModel::getRawDocument())