Document listenTo's implicit 'this' context.

That the context to the callback in listenTo is this, is only
documented as issue discussions on github. This adds explicit
documentation of that semantic.

See Issue #1999
Fix a small typo in patch.
Tweak context comment.
Fix #2535 - Update changelog with Model#set changes.
Merge pull request #2591 from braddunbar/model-set-changelog

Fix #2535 - Update changelog with Model#set changes.
Avoid double _validate calls in _prepareModel
Don't revalidate in Collection#_prepareModel