Fix bug, TypeError: unicode argument expected, got 'str' on core management validation
Traceback (most recent call last):
  File
"/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/arbiter.py",
line 485, in spawn_worker
    worker.init_process()
  File
"/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/workers/base.py",
line 100, in init_process
    self.wsgi = self.app.wsgi()
  File
"/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/base.py",
line 103, in wsgi
    self.callable = self.load()
  File
"/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/djangoapp.py",
line 101, in load
    return mod.make_wsgi_application()
  File
"/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/gunicorn/app/django_wsgi.py",
line 36, in make_wsgi_application
    if get_validation_errors(s):
  File
"/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/django/core/management/validation.py",
line 150, in get_validation_errors
    e.add(opts, "'%s' has a relation with model %s, which has either not
been installed or is abstract." % (f.name, f.rel.to))
  File
"/Users/avelino/.virtualenvs/yacows.virgula/lib/python2.7/site-packages/django/core/management/validation.py",
line 19, in add
    self.outfile.write(self.style.ERROR(force_str(u"%s: %s\n" %
(context, error))))
TypeError: unicode argument expected, got 'str'
Add myself in AUTHORS
Remove import not used, force_str on core management validation