diff 5.x-1.1

Changes since 5.x-1.0:

• #110857 by rotzi (with some changes by dww): complete re-write of the module. Now based on the GPL'ed mediawiki PHP diff engine from phpwiki. Instead of a separate diff tab, this overrides the revisions tab and provides radio buttons to select which revisions to view diffs between. Shows diffs in unfiltered, raw node data, instead of rendered HTML. Also, provides a hook_diff() for modules to define any parts of a node that they control with revisions (e.g. file uploads, etc) for display.
• #115226 by rotzi: add new 'Preview changes' button when editing a node.
• The revisions tab can now be disabled by node type.
• Improved help text.