Allow apps without models.py on Django <1.7

This wasn't officially supported by Django prior to 1.7 but is used by
some third-party apps such as Grappelli

This commit adds a somewhat contrived test app to avoid future
regressions by ensuring that the test suite always has an application
installed which does not have models.py

See #1073
Tolerate non-importable apps like django-debug-toolbar

If your installed app isn't even a valid Python module, haystack will
issue a warning but continue.

Thanks to @gojomo for the patch

Closes #1074
Closes #1075