Replace after_ callbacks with before_

Beginning of discussion: https://github.com/globalize/globalize/issues/325

I replaced after_create/after_update with before_create/before_update and set autosave: true.
Now we don't need to explicitly call translations.save inside save_translations!, because autosave takes care of it for us.
Test results haven't changed (192 tests, 1 skips).

Also I added a couple of tests for the current null: false case.

I merged my current branch into the previously created before_callbacks branch and I'm not sure that it would give the best git log. Can anybody validate it for me? (I can fork globalize again and reapply all changes by hand, if it's needed)