Deprecate NotifyingGenerator

[Vadim Gritsenko]

Hi All,

I'd like to deprecate Notifying handling in the sitemap engine. This  
would result in following deprecations:

  * NotifyingGenerator
  * Constants.NOTIFYING_OBJECT

There is ExceptionGenerator, introduced in 2.1.8, which replaces  
NotifyingGenerator. WDYT?

Vadim