Fix for #1826

Add Show/Hide toggle link for showing/hiding the explanation, because
adding/changing buttons leads to a dark path

	modified:   templates/exams/view_exam.html

@jinpa, @sefk, @jbau: Rather than changing the text of buttons to various things or adding a new button to clutter up the screen, I just have a toggle link in the explanation area that says "Show Explanation" and the explanation hidden. You get the idea. This seemed like the best way to handle it under the circumstances.

TODO: Refactor all this JavaScript after the DB class launches, as I'm aware of dup code.

Screenshots:

![Screen Shot 2013-01-12 at 1 15 29 PM](https://f.cloud.github.com/assets/931665/62313/9559f546-5cfd-11e2-94e9-c68beb4d6e78.png)

![Screen Shot 2013-01-12 at 1 15 46 PM](https://f.cloud.github.com/assets/931665/62315/9f0da60a-5cfd-11e2-87da-4c72b59963cf.png)