Merge pull request #22 from ferrix/identity

Fix email subject prefix and simplify code
Add support for API keys

Read API keys from environment with the names django-socialregistration
expects. This applies for:

* Twitter
* Facebook
* LinkedIn
Merge pull request #23 from ferrix/social

Add support for API keys
Loosen dependencies
Merge pull request #24 from ferrix/req

Loosen dependencies
New lint and fixes accordingly
Merge pull request #25 from ferrix/lint

New lint and fixes accordingly
Read allowed hosts from environment

Recent versions of Django disallow connections with target hosts
not listed in `ALLOWED_HOSTS`. This change separates the hostnames
from application code.