Added support for specifying multiple user model

Added ability to define multiple configurations for Mongoid::Userstamp.

```
Mongoid::Userstamp.config(:admin) do |c|
    c.user_model = :admin
    c.user_reader = :current_admin
end
```
Now this configuration can be used in a particular model like:

```
class Article
    include Mongoid::Document
    include Mongoid::Userstamp
    # specify the configuration to be used.
    mongoid_userstamp :admin
end
    
```

For the sake of backwards compatibility, following works as well (loads up the default configuration in this case):

```
class Article
    include Mongoid::Document
    include Mongoid::Userstamp
end
    
```

Issue can be referenced here: https://github.com/tbpro/mongoid_userstamp/issues/12