Remove observers et. al

Here's progress so far:
- removed observer support & tests
- ActiveSupport Logger default format changed; made tests care more about relevant details
- mimicked mass assignment sanitizing that was relied on to remove an :index key before model attributes were set

TODO:
- create a compatibility module to adapt mass-assignment method calls
- Dirty tests are still failing.  If you inspect the post.changes hash on spec/examples/dirty_spec.rb:227, instead of having the two comments it has two nils.  I haven't figured out yet why this is so.  Open to suggestions, but meanwhile will keep digging.