add 'goto top' link in lms while scroll not on top of the page

In LMS, how about add a button for easily scroll to top of the page while studying or doing problems if unit is more than one page.  If on the top already, then no show "goto top" button.

Use jquery plugin goToTop.

![screenshot14](https://cloud.githubusercontent.com/assets/3839631/3397691/76ac2ff2-fd23-11e3-8bee-b04fd4a142c4.png)