Fix icons and typos in ui tour

Fixes #6639 , or tries to. The x icon is too large still, and the forward/backward buttons aren't aligned correctly:

![image](https://cloud.githubusercontent.com/assets/83444/4538577/18bfc12c-4df2-11e4-837e-b9483e3ab488.png)

I imagine this should be something pretty easy to fix with the right CSS classes, but my knowledge of CSS is lacking.
If you take my suggestion below, you will need to modify `top` to 5 px and `right` to 14 px to make it symmetrical with the `next` button...