[core] Add Owner class
[core.Session] add get_owners() method to fetch owners
gca-client: add support to fetch owners
gca-client: extract common code to write json to stdout
[core.Session] Add support for fetching complete abstracts

Including owners for now, but will include stateLog data too
[core] Add LogEntry class for representing StateLogEntries
[core.Session] Add function to fetch state log
[core.Session] _complete_abstract: also fetch stateLog
gca-client: add command to fetch logs for a given uuid
gca-client: add support for fetching a full abstract with --full
[core.Abstract] Expose the state log as 'log' (if available)
gca-lint: Warning for abstracts without stateLog
[core.Abstract] expose owners as .owners if available
gca-lint: Report an error if abstract has now owners
[core.Session] use _build_url also for get_owners
[core.LogEntry] report timestamp back in datetime object

string is still available as timestamp_str
gca-client: new --path option for image command
[core.Session] use file cmd as fallback for image type detection