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, but the specified mappedBy association on the target-entity Ibw\JobeetBundle\Entity\Affiliate#categories does not contain the required 'inversedBy=affiliates' attribute.

Affiliate作为映射关系的拥有者，需要定义inversedBy。
Merge pull request #3 from justplaymore/patch-1

补全Affiliate和Category实体的双向映射关系定义