Moved logic to set sortable_foreign_key property to Sortable instead of SortableAdmin.
Added instance methods to get next or previous instance, optionally filtered by a sortable foreign key field.
Added option to disable sortable foreign key filtering.
Removed unused variable.
PEP8 cleanup.
Fixed missing kwarg in calls to get_next/previous.
Added basic test of get_next/previous.
Added Travis CI config.
Updated requirements and Travis config.
Added cd to Travis script.