Feature/issue description diffs

This alters displaying a change in an issue description from a long text of the schema:

<pre>
Description changed from Lorem Ipsum to Lorem Ipsum new
</pre>

to 

<pre>
Description changed (Details)
</pre>

Where Details is a link that when clicked loads a diff view from the server. The diff view is equal to that displayed for a wiki.

A lot of work has gone into refactoring the journal formatters. The OpenProject specific code has been factored out of the acts_as_journalized plugin and moved into lib. In addition, modifying the way a journal entry is formatted should now be easy.