Fixing how PickleProperty's are surface in the API.

Uses a string and leaves pickling/unpickling to the
property itself rather than forcing the user to do the
same.

Fixes #114.
Merge pull request #115 from dhermes/fix-pickle-property-handling

Fixing how PickleProperty's are surface in the API.
Adding symlinks to library in all examples.
Basic working app (before hacks).
Using patched version of endpoints.

I tried to subclass
  endpoints.protojson.EndpointsProtoJson
and then monkey-patch it on the protojson module but this
proved too difficult since there are so many explicit
references to the wrong class.
Hack to make sure repeated elements are not overwritten if not sent.