Fixes #142: Circular dependencies
Fixes #142: Circular dependencies

Bug introduced in #84

 1. The fixture is now registered in the array _before_ recursing.
 2. `class_exists` is now called so that the error about unexistant
    dependencies is properly thrown in `validateDependencies`.

plfort/data-fixtures,doctrine/data-fixtures,Tony133/data-fixtures,Werkspot/data-fixtures,oswaldderiemaecker/data-fixtures,gnat42/data-fixtures,alexsegura/data-fixtures,jenniferyien/data-fixtures,theofidry/data-fixtures,DACSoftware/data-fixtures