Copy the existing code from google-cloud-cpp

[coryan]

I probably should have created this earlier.

• Copy holder.py and its test
• Copy bucket.py and its test
• Copy object.py and its test
• Copy database.py and create a test for it
• Copy emulator.py and create a test for it, maybe rename to testbench.py
• Copy grpc_server.py and create a test for it

[coryan]

We decided not to move the storage/v1 gRPC implementation. I can now close this issue.