Fluid scrolling buttons

This fluidifies the positionning of the follow the logs button.
We're relying on the browser to define it's position instead of setting the top at each scroll.

Also, as suggest by @joshk in #89, if the build has finished running, it renames the button content to "Scroll to end of logs".

Finally, as per #89, it makes the "back to top" button follow the scroll too.
This button now looks like the following:
![screen shot 2013-12-05 at 16 05 45](https://f.cloud.github.com/assets/9347/1683510/c5fcc440-5dbe-11e3-88a1-62e8fa0a1d29.png)

As a side note, this back to top button might need a bit of redesign, since it now might look weird if the log has a full length.
![screen shot 2013-12-05 at 16 06 18](https://f.cloud.github.com/assets/9347/1683519/f210142e-5dbe-11e3-8a9f-08335ab350aa.png)