Added warning when user tried to create model with pluralize name.

This PR will generate singularized model and its corresponding files even if user(specially newbies) mistaken provide pluralized model name.

```
Before

$ rails g model posts title:string
      invoke  active_record
      create    db/migrate/20120621124605_create_posts.rb
      create    app/models/posts.rb
      invoke    test_unit
      create      test/unit/posts_test.rb
      create      test/fixtures/posts.yml

After

$ rails g model posts title:string
Plural version of the model detected, using singularized version. Override with --force-plural.
      invoke  active_record
      create    db/migrate/20131228115512_create_post.rb
      create    app/models/post.rb
      invoke    test_unit
      create      test/models/post_test.rb
      create      test/fixtures/posts.yml
```