Moved caching in AbstractHolidayCalendar to the instance level
Closes #9552

![image](https://cloud.githubusercontent.com/assets/881019/6389208/060f4ecc-bd9a-11e4-8e20-6af34fb90c3d.png)
 @dhirschfeld this looks good. Can you add a release note in Bug fixes, and squash. ping me when pushed.
 @jreback - added a comment in `whatsnew/v0.16.1.txt`. Let me know if that's ok.
 @dhirschfeld looks good. pls ping when green.
 @jreback - looks like travis finally got there in the end!
 @jreback - rebased after conflicts in `whatsnew/v0.16.1.txt`. Will ping when travis says ok
 thanks @dhirschfeld 
