Reduce complexity of Post - introduce a PostAnalyzer class

This follows on from https://github.com/discourse/discourse/pull/910 and @eviltrout's suggestion to create a `PostAnalyzer` class that replaces the `PostAnalyser` module and encapsulates `cooked_document` 

`PostAnalyzer` has its own specs - these are pulled out of `post_spec.rb` so that no specs have been added or removed from the project.