Improved EPUB Tests

Rewrote features test to remove all unimplemented features.

There are now all three examples of where an image can be included in
the test.
  1. Cover image
  2. As a spine elemnt
  3. In the document

Tests have also been added to make sure that the mediabag contains all
these images after processing.
Merge pull request #1512 from mpickering/master

Improved EPUB Tests