RFCT Implement aggressive-unload with a hook

This makes the inner loop simpler.
TST Check that file_store.list works

Currently it does not work correctly in Python 3.4 as there is a mixup
of unicode and bytestrings.

This is the root cause of issue #10
ENH Fix list return in Python3+ to return bytes

This causes many problems in Python3+.

Closes #10

It also fixes a more serious bug whereby all of the keys in a jugdir
started with b'! This was because a conversation from bytes to str was
causing the prefix b' to be added to all keys!