Improved filter support

This patch was inspired by #16 and need to make filters work with mongoid 2.4.

Features added by @lukaso :
* search fields marked as field
* will allow arbitrary fields, but only as string, as mongo is schemaless

Features added by me:
* filters works also with mongoid 2.4
* added support for input types other than string, as mongoid let us define types and associations

If somebody wants to use it before it is merged or just test it:
```ruby
gem 'activeadmin-mongoid', :git => 'git://github.com/Ostrzy/activeadmin-mongoid.git', :branch => 'filter_support'
```