Fix conditions in #showEm()

* return when the pOriginal is null, not when it is non-null
  (Eclipse noticed a definitive NPE)
* return when running in headless mode (unit tests via maven)
Avoid NPE when the expected tree is indeed null
Remove unneeded @SuppressWarnings("unchecked")
Merge pull request #46 from ankon/imageio-jpeg-test-warning

Remove unneeded @SuppressWarnings("unchecked")
Merge pull request #44 from ankon/common-image-null-reverse

Fix conditions in #showEm()
Merge pull request #45 from ankon/imageio-jpeg-test-null

Avoid NPE when the expected tree is indeed null
Code clean-up after merge + Rewrote test to use JUnit 4.