Possibility to show uploaded image on a testlist

Issue 126

Possibility to show images on a testlist. Features:

• Model Test: a boolean field 'display image' is added. Field is only visible for upload.

• The imghdr module checks if a uploaded file really is an image

• Images are shown under the testlist, this avoids clutter in the form when using multiple images

• jQuery is used to add the image interactively

• The images are shown during perform, edit and review of a testlist

• The name of the test is shown above the image

• The size is max 1000px * 1000px