Show diff for issue description change
This patch closes both #328 (put the fact of description change to issue history) and #746 (visualize description change).
It goes rather simple path and relies on availability of 'diff' utility (not a problem in many cases).
I just upgraded JournalDetails' values to :text field type (from :string of 256 symbols); better approach might be introducing Issue versioning with acts_as_versioned and making JournalDetails refer to previous and next versions of issue; but I thought this would be too heavy for this patch and better would be another improvement (if needed).
Contributed by Texuna Technologies.