gkeys: Add --timestamp option 

Add timestamp and dest options to the verify action.
gkeys/lib.py: Py3 compatibility changes
gkeys/actions.py: Fix loading of multiple keys in one keydir in installed()
gkeys: Refactor fetchseed operation

Use the Action.verfify() to download the seeds, .sig and use the timestamp files.
gkeys/actions.py: Make the '.sig' file extension the first tried (detached sig ext.)