Removing safety section from idempotency topic.  Was duplicate text.

On the idempotency lesson, there were two sections, Idempotence and Safety.  The Safety section was just the Idempotence one duplicated.  I removed the Safety section and updated the Idempotence section to be more clear.  

One other thing I did was to remove the reference to "setter" methods in programming languages.  I think that will just confuse people.
![tmp](https://f.cloud.github.com/assets/5596986/1471376/0d231e10-45e0-11e3-94d9-9f87144b4d0c.png)