Adds static_domain kwarg to EmailGenerator to generate random e-mailaddresses with the same domain

This feature will allow users to generate random e-mailaddresses with a static domain; e.g. noreply.com

Example usage:

In autofixtures.py:

    from autofixture import AutoFixture, generators, register,  
    
    class AccountAutoFixture(AutoFixture):
        field_values = {
            'email': generators.EmailGenerator(static_domain='noreply.com'),
        }
    }

    register(Account, AccountAutoFixture)