refactor(theme): remove angular2-toaster styling

[Tibing]

Please read and mark the following check list before creating a pull request:

• [] I read and followed the CONTRIBUTING.md guide.
• [] I read and followed the New Feature Checklist guide.

Short description of what this resolves:

BREAKING CHANGE:
angular2-toaster styles were removed from nebular.
You have to use NbToastrService instead.

• Add NbToastrModule into imports of your root module.
• Inject NbToastrService into the required component.
• call NbToastrService.show(...) to render toasts.

For more information, check toastr documentation.

[nnixaa]

@Tibing we need to describe the breaking change and how to replace 3rd party toaster with the Nebular one.

[Tibing]

@nnixaa, done