Documentation and cleanup of automatic discovery of inverse associations

Removed `:automatic_inverse_of` in favor of using the `inverse_of: false` to tell us whether we don't want to use automatic inverses. Changing the documentation and adding a CHANGELOG entry for the automatic inverse detection feature.

@jonleighton I don't think it's actually possible to use `inverse_of: nil` to tell us to not automatically find inverse associations. This is because the feature would be completely useless, since by default, we have `options[:inverse_of] = nil`. This means, by default we do not use automatic inverse finding.

Conversely, having the user specify `options[:inverse_of] = true` to use automatic associations doesn't make sense because at that point, the user can make the extra couple characters and type in the inverse.