Feature/464 contact field

This pull requests solved #464 and #468 : An employer is now able to fill in his contact data in extra contact fields. By that the employer can be reached directly via email/phone or even mail. 
Furthermore there are now contact fields for each job offer which are pre-filled with the contact data of the employer.
If you want to seed a contact you should not fill that information out manually but instead just set the contact.counterpart with an existing employer. If for any reason you would have to do so however, you should use an existing id of 1 or higher and use the capitalized class name 'Employer'.