adjust indent to 4 spaces
refactor storage to storage_common; fix localstorage.py indent

take the stream_generator() func from irodsstorage and localstorage
and put it in storage_common as read_stream_generator.
The original idea was that different backends would need different
functions, but at least for these two, it's the same.

fix indent in localstorage.py for 4 spaces
add get/set metadata func stubs in localstorage.py