email subject as setting

# Issue
When I was incorporating this app to my project I discovered that the email subject is coded within the models send_login_email method. The subject 'Login code' would likely be confusing to my user base.

# Challenge
I started to subclass the model within the model that my app is using django-nopassword as authentication. However it seems to me that's a fair amount of effort just to change the subject of an email.

# Solution
I decided that at lease for me it would be best that subject be configurable via settings.py. 
## Changes Made
- [x] Fix `subject` to read the settings value `LOGIN_EMAIL_SUBJECT` w/ default value
- [x] Fix README informing users of available setting