#ignore_undefined_method_for_nil setting to show in-cell errors in tables

This patch let us avoid page crash in case of missing association or nil value.. (pretty annoying sometimes)

```ruby
ActiveAdmin.setup do |config|
  config.ignore_undefined_method_for_nil = true
end
```

![screen shot 2013-12-05 at 12 04 25 pm](https://f.cloud.github.com/assets/26019/1680314/fea4c62c-5d72-11e3-8457-c2b08e02723b.png)

----
and if click then description appears

![screen shot 2013-12-05 at 1 01 45 pm](https://f.cloud.github.com/assets/26019/1680315/03c2e06c-5d73-11e3-982a-a1d5688005a7.png)