Add Fabricate.clear_singletons! utility method

Closes #7

LGTM given by: @marten

Squashed commit of the following:

commit 1f1b31c9ca77e18c0a520906c41ddd5c796e2fd1
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Mon Jun 16 22:46:08 2014 +0200

    update change log

commit 0e59e14293e0e90367d2e95c3ecefe5edefeb7a0
Author: Samuel Esposito <s.esposito@roqua.nl>
Date:   Mon Jun 16 22:38:00 2014 +0200

    add clear_singletons! method