Tailer tweaks

[The PaaS issue](https://git.hubteam.com/HubSpot/PaaS/issues/25) (please discuss it there) brings up the problem that the "To X" buttons hide the text behind them.

The tweaks involve fading out the tailing indicator and the buttons (unless you hover over them) if you're hovering over the last 5 lines.

Example: http://imgur.com/a/tsYIN