Fix #83:  Add 'exclude_from_indexes' method to Entity.

Set it via ctor argument.

Pass it to `Connection.save_entity`.

Fields in the sequence will have the `indexed` field set False in the
corrsponding protobuf.

Fixes #83.
I think 'str' is correct:  the field names are always "native" strings (only values are "text" vs. "bytes").