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>

offlineimap/repository/Base.py

@@ -103,9 +103,11 @@ class BaseRepository(CustomConfig.ConfigHelperMixin, object):
     def getmapdir(self):
         return self.mapdir
 
+    # Interface from CustomConfig.ConfigHelperMixin
     def getsection(self):
         return 'Repository ' + self.name
 
+    # Interface from CustomConfig.ConfigHelperMixin
     def getconfig(self):
         return self.config