introduces ajax triggered inline diff on work_package#show

The functionality itself is adapted from issue#show.

On the other hand, the diff is no longer rendered with the full url as the
link's href. Using the full url caused problems with selenium tests that
ran into problems with cross domain requests. This could have been fixed
by setting the application's host and protocol setting accordingly.
However, it is much cleaner to use a relative url. So for rendering the
diff inside the application the behaviour was alterd to use relative
url. The full url is now only used where it is absolutely necessary
(pdf, email).

Please enter the commit message for your changes. Lines starting
more general implementation for diff view

This view should not be specific to issues/work_packages.

Using polymorphic_url will for now lead to paths like "/issues/5".
But once only work_packages exist this will no longer be a problem.
updates changelog