Support for custom model names

I think that custom model names (what is displayed in menu and in controls) would be very useful.

This implementation is just a prototype, I am curions what you think about this feature.

I think it could be easily integrated with "configuration" feature implemented in #36 later.

This implementation allows setting of custom names via 2 ways.

```ruby
# user_persistance.rb
class UserPersistance
  def self.upmin_name
    'User'
  end
end
```

or

```ruby
# initializers/upmin.rb
Upmin::Model.model_names = {
  UserPersistance => 'User',
}
```