Add a setting to specify the email field length.

Provide a setting to override the max_length for the email field.

The default is kept to be the same for the django EmailField, which is 75.

This now allows the user to override the default to a more standards compliant 254 (see standards comment here: https://docs.djangoproject.com/en/dev/ref/models/fields/#emailfield)

Interestingly when running the tests (on ubuntu with python 2.7.5), I had import errors from
unittest.mock, and I'm not sure why.