Rename mergetool() to merge() to be simmilar to diff()

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Remove unused function

Signed-off-by: Tomas Hozza <thozza@redhat.com>
All Differ methods are now instance methods, not classmethods

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Add string representation for Differ

Signed-off-by: Tomas Hozza <thozza@redhat.com>
Add get_supported_tools() method for Differ

Signed-off-by: Tomas Hozza <thozza@redhat.com>