Offlineimap duplicating email redux

[Sebastian Spaeth]

I have dug out my branch again and rebased it against current master. It
consists of 2 small patches creating convenience functions and the main patch.

02252dc folder/Base.py: Add getmessageuidlist() as a helper function
3c1da55 folder/Base.py: Implement uidexists helper function
f1fe844 Revamp Syncing strategy.

The last one is a bit big (2 files changed, 181 insertions(+), 166
deletions(-)) I am sorry for that. I tried to make it smaller, but the
changes were so interdepended that I gave up. The additions come really
from the code comments that I inserted.

It would be great to get feedback/testing on that code. Now that I
resurrected it, I made a backup of my maildir and will be running it for
a while...

Sebastian