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/accounts.py

@@ -78,6 +78,7 @@ class Account(CustomConfig.ConfigHelperMixin):
     def getlocaleval(self):
         return self.localeval
 
+    # Interface from CustomConfig.ConfigHelperMixin
     def getconfig(self):
         return self.config
 
@@ -90,6 +91,7 @@ class Account(CustomConfig.ConfigHelperMixin):
     def getaccountmeta(self):
         return os.path.join(self.metadatadir, 'Account-' + self.name)
 
+    # Interface from CustomConfig.ConfigHelperMixin
     def getsection(self):
         return 'Account ' + self.getname()