Add Post Tags and their relations

This PR adds Tags field to the Blog Post form. See below

![relations](https://cloud.githubusercontent.com/assets/2228236/3942432/dc976886-2564-11e4-8721-438f4a2d5d35.png)

Since it relays on ManyToMany relations and there is no annotation available for this in the CMS
the relations are assigned from initialize() methods. Related PR: https://github.com/PhalconEye/cms/pull/78