补全Affiliate和Category实体的双向映射关系定义

Symfony 2.5.*， 查看debug信息，在Mapping errors里出现如下错误

>     The field Ibw\JobeetBundle\Entity\Category#affiliates is on the inverse side of a bi-directional relationship,  the specified mappedBy association on the target-entity Ibw\JobeetBundle\Entity\Affiliate#categories not contain the required 'inversedBy=affiliates' attribute.

Affiliate作为映射关系的拥有者，需要定义inversedBy。