attempt 1 to learn Unicode (unifuncs)

[Nicolas Sebrecht]

I call this branch "unifuncs". It introduce the "uni" module with simple
functions handling the encoding conversions.

I don't expect a full review. It's large and quiet boring sometime. The more
intersting parts are the patches 1/4 and 3/4.

The diffs online: https://github.com/nicolas33/offlineimap/compare/OfflineIMAP:next...ns/unifuncs?expand=1
The branch TIP: https://github.com/nicolas33/offlineimap/commits/ns/unifuncs

[PATCH 1/4] docs: API: introduce unicode module
[PATCH 2/4] add IMAPClient embedded module to work with IMAP UTF-7
[PATCH 3/4] introduce utils/uni.py module
[PATCH 4/4] learn unicode support