Issue #2641546: Allow options to be sorted by value

Squashed commit of the following:

commit 678e72662babd01575e475f1482eb61c556beb64
Author: legolasbo <legolasbo@2480548.no-reply.drupal.org>
Date:   Tue Dec 29 23:35:24 2015 +0100

    Add test coverage for the newly added settings.

commit ac442bc88e34af5934ecc3ddbb8bd02e839af78a
Author: legolasbo <legolasbo@2480548.no-reply.drupal.org>
Date:   Tue Dec 29 23:21:37 2015 +0100

    Add sorting functionality with test coverage

commit cebab93ebcf0bcf874dcc0316c4efc130c22c6ff
Author: legolasbo <legolasbo@2480548.no-reply.drupal.org>
Date:   Tue Dec 29 21:50:12 2015 +0100

    Allow sorting to be configured trough the settings form

commit 224c16b822e9ead150a0483c501da747571742a1
Author: legolasbo <legolasbo@2480548.no-reply.drupal.org>
Date:   Tue Dec 29 21:49:42 2015 +0100

    Update schema