Minor updates

Ciao! I forked your gem and made some little enhancements:

* Added a Search box for Banners in Admin
* Refactored Helper and Configuration class
* Some code cleanup

I'm testing a feature to attach the banner to a Taxon, if you find it useful I could make a pull request for that too.

Also, I created a 2-0-stable branch to keep master for Spree 2.1, I think you should do this in your repo.