Merged LoginView post() method into form_valid() and form_invalid()

Since post() had the same code as the superclass, the custom code has
been moved inside the form.is_valid() condition to form_valid() and
form_invalid().

Also, form_valid() calls the superclass instead of having the same
HttpResponseRedirect.
Merge pull request #4 from jcugat/loginview_methods

Merged LoginView post() method into form_valid() and form_invalid()