Brought CustomConfig.py into more proper shape

 - Multi-line documentation for functions and methods
   now has ending triple-double-quotes on an own line,
   as per PEP 257.

 - Added documentation and comments to almost all functions
   and methods.

 - Added stub implementations for getconfig() and getsection()
   inside CustomConfig.ConfigHelperMixin to provide sane
   run-time diagnostics for classes that doesn't implement them.

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Update imaplib2 to 2.37

The only fix is
 - add missing idle_lock in _handler().

Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>
Imaplib2: trade backticks to repr()

Backticks are gone in Python 3.x.

GitHub issue: https://github.com/OfflineIMAP/offlineimap/issues/121
Signed-off-by: Eygene Ryabinkin <rea@codelabs.ru>