Add sample with a custom NDB property --> ProtoRPC field mapping in registry

[dhermes]

Add sample with a custom NDB property --> ProtoRPC field mapping in registry

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_{Code Hosting Comment Metadata:} sub>author.htmlLink=https://code.google.com/u/dhermes@google.com/</sub sub>author.name=dhermes@google.com</sub _{id=0 published=2012-12-10T17:08:01.000Z}

_{Code Hosting Issue Metadata: published=2012-12-10T17:08:01.000Z stars=1 updated=2012-12-11T20:02:45.000Z originalIssue=https://code.google.com/p/endpoints-proto-datastore/issues/detail?id=17} sub>author=@dhermes</sub

[dhermes]

_{Code Hosting Comment Metadata:} sub>author.htmlLink=https://code.google.com/u/dhermes@google.com/</sub sub>author.name=dhermes@google.com</sub _{id=1 published=2012-12-11T20:02:45.000Z updates.labels=[   -Type-Enhancement,   Type-Sample, ]}