Filter support

This patch adds support for filters which were previously disabled.

Features:

* search fields marked as field
* will allow arbitrary fields, but only as string, as mongo is schemaless

Pull request now done as topic branch.

Fixes based on feedback in #15.

For those that wish to use the fix until the request has been merged:

```
gem 'activeadmin-mongoid', :git => 'git://github.com/simplybusiness/activeadmin-mongoid.git', :branch => 'filter_support'
```