Added test for valueview experts in general and StringValue expert specifically

Introduces jQuery.valueview.tests.testExpert which will test a Expert constructor on basis of a
given test definition. Added first expert test using this new function on the StringValue expert.
Fixes some minor issues and updates documentation of jQuery.valueview.Expert itself to pass the
new tests.

Change-Id: Ia681d6a8a31771cc000d5f1f423de25a77f6a6c2