Add "Contrast" image effect

And here is the patch... cross your fingers that the color values in the test are valid ;)

Here is a new patch with the following additional changes:

* Alphabetically sorted list of effects in README.md
* Changed t() to $this->t() (also for Brightness effect - I know this is out of scope of this issue, but let's get that fixed fast, hehe)

@mondrake: The '0x' you mentioned is required for the correct value of the ImageMagick command, as it represents the brightness change (0 = none). You can find more information at: http://www.imagemagick.org/script/command-line-options.php#brightness-co...

Unfortunately the test will fail again, because of the wrong test color results and I am working on that. But unfortunately I can't get my test to work on my local machine, bacause of strange errors thrown in the ImageEffectsTestBase class. So perhaps there will be another ticket fixing this before I can examine my tests locally ;)

Fail      Completion ImageEffectsContr   30 Drupal\image_effects\Tests\ImageEff
    The test did not complete due to a fatal error.
Fail      Other      ImageEffectsTestB   44 Drupal\image_effects\Tests\ImageEff
    Unable to install modules image, image_effects, simpletest due to missing
    modules image_effects.
Fail      Role       ImageEffectsTestB   63 Drupal\image_effects\Tests\ImageEff
    Invalid permission administer image styles.
Exception Recoverabl WebTestBase.php    600 Drupal\simpletest\WebTestBase->drup
    Argument 1 passed to Drupal\simpletest\WebTestBase::drupalLogin() must
    implement interface Drupal\Core\Session\AccountInterface, boolean given,
    called in ImageEffectsTestBase.php
    on line 64 and definedDrupal\simpletest\WebTestBase->drupalLogin()
    Drupal\image_effects\Tests\ImageEffectsTestBase->setUp()
    Drupal\image_effects\Tests\ImageEffectsContrastTest->setUp()
    Drupal\simpletest\TestBase->run(Array)
    simpletest_script_run_one_test('82',
    'Drupal\image_effects\Tests\ImageEffectsContrastTest')

Whoops, I hid the wrong file!

Fixed typo in issue description!

Here are the updated patches (unfortunately still with failing test, but I will have a look at that now) for reference.

@mondrake: I really don't know why there were these whitespaces ;) I removed them in both contrast and image effect now.

Just came back from the new Star Wars movie and I hope the force is strong with me right now ;) So let's give it another try!!!

Here is a patch with adjusted test color values. And for anybody interested in the local test problems mentioned in #5: It is not possible to test a module installed in an install profile folder ;) So it has to reside in docroot/modules otherwise it won't be found, because simpletest uses the testing profile during test runs.

So... and finally here is another new patch with adjusted test color values for ImageMagick toolkit.

Cool, thanx! ;)

I guess the different color values need to be addressed as is, because both toolkits handle contrast changes completely different. ImageMagick even has another contrast method -sigmoidal-contrast (http://www.imagemagick.org/script/command-line-options.php#sigmoidal-con...), which increases the contrast without saturating highlights or shadows.