Django 1.10 support for FieldTracker

[jarekwg]

Fixes #232

So two things going on here.

First, as of Django 1.8, we should be using get_deferred_fields rather than manually probing for isinstance(field_obj, DeferredAttribute).

Second, as of Django 1.10, it seems inherited models don't pull through the deferred fields in __class__.__dict__. Have to consult the base class to get them. I have a feeling this might be a regression and will probs raise a ticket with django to make sure, but this bandaid should work for the time being. There's a slight niggle in that dj110 now supports overriding fields from the base class. I think I'm building the dict in the correct order so that stuff is overridden correctly, but there might need to be additional tests added to check for this (can open a new issue). Also, perhaps there's a way to get a hold of the deferred field that avoids the use of __class__.__dict__ altogether?

Tests all pass now so removed dj110 from allowed failures.

[jarekwg]

After some django digging, a handle to DeferredAttribute is now obtainable by accessing the field through the instance's class. I've updated the PR to use this implementation, as it's considerably neater.

[carljm]

I'd rather leave the trunk builds in allow_failures even if they pass currently. Once we get into the heat of 1.11 feature development, they will probably break again, and I don't want the build breaking due to changes outside model-utils' control. Django trunk build is just advance warning, it's not a blocker.

[jarekwg]

Oh True, will do!