Change help link in header to a dropdown

Per TLC request - they've created Instructor and Student Help shells in Canvas.

Looks fine in Chrome and Safari. Looks a bit goofy in Firefox and IE, but servicable. You can't reliably style `<select>` elements so this is as good as its going to get without spending more time than it's worth on it.

 I threw an IE 7 screenshot in there; even though it isn't supported and users get a warning when the log in, it's still the 3rd most popular version of IE according to our stats (16th overall).

Chrome:
![chrome](https://f.cloud.github.com/assets/146111/1853588/8b838344-770e-11e3-84fe-7f5690eb22b8.png)

Firefox:
![firefox](https://f.cloud.github.com/assets/146111/1853589/8b83adce-770e-11e3-9029-1096391e350a.png)

Safari:
![safari](https://f.cloud.github.com/assets/146111/1853593/8b890b66-770e-11e3-8b75-2a5b0f1dc2cf.png)

IE 10:
![ie 10](https://f.cloud.github.com/assets/146111/1853592/8b887e94-770e-11e3-82ec-ec9de5aa0e2f.png)

IE 9:
![ie 9](https://f.cloud.github.com/assets/146111/1853591/8b85eb48-770e-11e3-90d9-d745725e13f4.png)

IE 7:
![ie 7](https://f.cloud.github.com/assets/146111/1853590/8b841d04-770e-11e3-8f3f-4670b85a04bf.png)