removes outdated comment
adds a journaled_class method

That way we do not have to load the class or do some magic parsing names when wanting to now what is journaled.
adds a very simple cache to cache arbitrary model objects

yields a block with object_klass and object_id to be able to cope
with not finding an object.
allows journal formatters to receive options and uses it to be able to
pass an object cache

For now it is only used by the NamedAssociation Formatter. The formatter
requires to read a lot of model objects.
Merge pull request #25 from opf/perf/journal_caching

Perf/journal caching