Make generator namespace aware

Currently the uploader generator is not namespace aware.
```
$ rails generate uploader MyModule::Avatar
create  app/uploaders/avatar_uploader.rb
```
In the above situation the uploader class is `MyModule::Avatar` and is defined properly in the generated file. But the path of the generated uploader class is wrong, it should have been `app/uploaders/my_module/avatar_uploader.rb`