Feature/port access

[xinyuli1204]

1. Allow pipeline to access the index of port if there are multiple outputs. eg. n["output"][1]
2. Add test code "port-access"
3. Add building blocks : Camera2 (output 2 ports, one image per port) and CameraN (output one port , n images per port)
4. Fix "parse_url" to handle both https and http request.