Uniform settings handling

For MANDRILL_API_KEY (e.g.,), look for these settings:
* ANYMAIL = { 'MANDRILL_API_KEY': '...' }
* ANYMAIL_MANDRILL_API_KEY = "..."
* MANDRILL_API_KEY = "..."

(the "bare" third version is used only for settings that
might be reasonably shared with other apps, like api keys)

anymail/exceptions.py

@@ -1,4 +1,3 @@
-from django.core.exceptions import ImproperlyConfigured
 import json
 from requests import HTTPError
 
@@ -68,10 +67,6 @@ class AnymailError(Exception):
         return description
 
 
-class AnymailImproperlyConfigured(AnymailError, ImproperlyConfigured):
-    """Exception for configuration problems"""
-
-
 class AnymailAPIError(AnymailError):
     """Exception for unsuccessful response from ESP's API."""