Move some models to a new instance app to avoid circular dependencies

[mhl]

There was an unnecessary circular dependency between the 'nuntium' and 'contactos' Django apps:

contactos.Contact needs nuntium (for nuntium.WriteItInstance) nuntium.OutboundMessage needs contactos (for contactos.Contact)

This wouldn't be a big deal, except that when we're migrating to Django 1.7 it means that makemigrations generates multiple initial migrations (0001_initial and 0002_auto..., the latter of which adds the problematic foreign keys). The second of these migrations didn't work properly on Django 1.7 and SQLite, so the easiest way to deal with that upgrade seems to be to:

• Remove the circular dependencies with South migrations and Django 1.6
• Then do the upgrade to Django 1.7, when we should now be able to have single initial migrations

This pull request deals with the first of those by creating a new 'instance' app, which contains:

WriteItInstance Membership WriteItInstanceConfig WriteitInstancePopitInstanceRecord

[coveralls]

Coverage decreased (-0.02%) to 98.451% when pulling 2c98be19a123e0f6a20d705bf21e9b80edbd78c7 on remove-circular-dependencies into d3b833b71923be7da5bb56d71426b224e6d2dd64 on master.

[chrismytton]

Tried this with sqlite and mysql and it's working as expected. So :+1: from me.