Add redis orm Ohm support

Ohm is a library for storing objects in Redis, a persistent key-value database. It includes an extensible list of validations and has very good performance. 
I add the Ohm support to database_cleaner and now it pass all spec and cucumber scenarios, it seems work well for me now.

Note:
* With orm Ohm, :only and :except accept options like "Widget:Name*" or "*:id*" that redis command KEYS accept
* We can configure connections specified as redis url, for example
<pre>DatabaseCleaner[ 'ohm', {:connection => 'redis://localhost:6379/1'} ].strategy = :truncation</pre>