Tidy admin area

This is the same as my PR for the previous reop - see https://github.com/priviterag/spree_slider/pull/33

Make admin area more consistent with other spree admin views. See screenshots of updated views:

![screen shot 2014-09-26 at 10 56 10 pm](https://cloud.githubusercontent.com/assets/145042/4420712/b01efafc-457c-11e4-9371-a0bd55172c62.png)
![screen shot 2014-09-26 at 10 55 47 pm](https://cloud.githubusercontent.com/assets/145042/4420714/b0271ee4-457c-11e4-89b0-5eeaeb0dd792.png)
![screen shot 2014-09-26 at 10 55 35 pm](https://cloud.githubusercontent.com/assets/145042/4420713/b0263330-457c-11e4-9647-5f7d7cdd4f76.png)

PS thanks for this gem, it's great!