fix(MR-16): Use cache for FileField and ImageField

[TrangPham]

So this took a very long time to get to. Here's the previous history #16. This is related to #8

That worked nicely for ManyToManyField but not for the ImageField or FileField.

This MR tries caching (which works for FileField and ImageField, but not ManyToManyField).... thus a combined solution is used. Cache is used primarily, but for any m2m/generic relation fields data it looked up from request.POST (sent via hidden embedded form) and saved the related models.

Note: Had to bypass _save_related and save_m2m but calls the underlaying functions

Tests have been added for ManyToMany, File, and Image fields and caching

[TrangPham]

Previously in 0.2.3.dev6 relied mostly on the cache and handled m2m fields as a special case.
Now in 0.2.3.dev7 relying mostly on the post data and using cache to treat files and images as special cases.

Also added handy dandy tests readme to outline all the mutations that should be covered in test cases.

🤞 That this works better because .dev6 introduced more issues than it fixed.

[TrangPham]

0.2.3.dev9 is on testpypi. See: #8 (comment) For more details of differences between dev7 and dev9

[TrangPham]

change_data for add and change_data for change could be refactors into helper methods to make this easier to read. Perhaps create a utils file for these Or add them to the existing one.

This would also also for better unit testing

[TrangPham]

Maybe 🤔 create a _assert helper method for this. It's required in most tests, but... it's bad practice to have any assert which is not explicit in the test func body.