base support for custom ransackers

## Given:
```ruby
class User < ActiveRecord::Base
  # attribute email
  ransacker :fullname do
    Arel.sql("CONCAT(`firstname`, ' ',`lastname`)")
  end
end

ActiveAdmin.register User do
  filter :fullname
  filter :email
end
```

## Before this PR ActiveAdmin only renders a filter field for email, not for fullname:
![before_pr_without_fullname](https://cloud.githubusercontent.com/assets/165599/4777993/6ca6710a-5bda-11e4-9661-73722a00a032.jpg)

## After this PR ActiveAdmin provides a search field for fullname:
![after_pr_with_fullname](https://cloud.githubusercontent.com/assets/165599/4777994/80edf368-5bda-11e4-9157-96512699d333.jpg)

I'm working on even more support for custom ransackers.