Add region support to AMWG plot set 8

These plots really need axis labels for clarity.

Global plot:
![set8_ann_trefht_ncep](https://cloud.githubusercontent.com/assets/6811403/5514225/a202aff8-87db-11e4-8ddb-397655e40e58.png)

"Southern extratropics" version:
![settesting_ann_trefht_ncep](https://cloud.githubusercontent.com/assets/6811403/5514231/bf3ce6b0-87db-11e4-890c-4e7b93889590.png)