simplify wrapper_classes_raw

```ruby
classes = nil
[*classes]
# => []
classes = "foo"
[*classes]
# => ["foo"]
classes = ["foo", "bar"]
[*classes]
# => ["foo", "bar"]
```
correct and simplify compatibility in README
Merge pull request #1096 from timoschilling/simplify_wrapper_classes_raw

simplify wrapper_classes_raw
derive collection from enum if declared for the method
use wrapper EnumOption objects intead of arrays (better comparison)
default to SelectInput for enum columns
ensure that the right option is marked as selected/checked for enum collections
no need for EnumOption class, the problem isn't that complex
enum options can now be translated with i18n
add docs to SelectInput
extract collection_from_enum?
raise an error whn trying to use enum on CheckBoxesInput (no multi-choice)
shift into a 'single choice' context
raise an error for enum on multi-select
add coverage of RadioInput support for enum
add documentation for enum on select and radio inputs
version bump to 3.1.0 final
CHANGELOG update
let's try indenting the CHANGELOG
fix stubs: Post#statuses should have been Post.statuses