Added _insert for App Engine

Returns a dictionary with processed values without applying the changes to datastore.
Now .insert uses _insert for data representation
Usage: this can be used for pre-processing data before lower level I/O (i.e. using google api for storing entities)

Discussion: https://groups.google.com/d/msg/web2py/lFKiIGary0c/h5jxr1gxiVoJ