[1093] HABTM Select Filters

`For #1093

+ Assuming Foo HABTM Bars, the below now works as you'd expect:
```ruby
    ActiveAdmin.register Foo do
      filter :bars
    end
```

+ `filter :attr, as: :select` now utilizes `ActiveRecord#pluck` when using Rails >= 3.2 (resolves #2123)
+ polymorphic associations are now added to the default AA filters for a resource