First commit, after modifying codes based on design doc feedbacks 7/20

[George-Wu]

Pre-commit for Dicom io connector
The dicomio.py supports three different Ptransform:

1. Qido Search request
2. Convert pubsub message to Qido input
3. Store a instance to Dicom store

Todo:

1. Add stream file objects supports to store operation.
2. Add pagination to client.
3. Finish unit test and push it.
4. Finish IT test.