Added support to models without database table

Hi, 
we needed humanizer for a model without database table and we made this change to support it.
This could be useful, for example, in contact forms that only send mail.

*Instruction*
To use this option just add in the model:

```ruby
include Humanizer
require_human_on
```

without specifying no method.

Hoping to please.

NebuLab Team