Bug 1089835 - Improve readability of Builds/Tests/Shortcuts in Help

This work fixes Bugzilla bug [1089835](https://bugzilla.mozilla.org/show_bug.cgi?id=1089835).

Nothing major, but I made some minor readability tweaks for the Help page. In it we provide some white space between the header item and its description, for the three text-based tables. While I was there I adopted the alternate row coloring in Treeherder itself, to help readability.

Here's the before:

![helpcurrent](https://cloud.githubusercontent.com/assets/3660661/4799703/be848a84-5e1e-11e4-8f50-6ff6175d7eaa.jpg)

And here is the after:

![helpproposed](https://cloud.githubusercontent.com/assets/3660661/4799709/c612e304-5e1e-11e4-8518-1693c130159d.jpg)

Similarly the shortcuts gets the same treatment, and the next/previous are unified on to a single line each. It was actually my initial goal and I got carried away :)

![helpproposedshortcuts](https://cloud.githubusercontent.com/assets/3660661/4799565/7a1c9a86-5e1d-11e4-888e-b4cb318a5ed9.jpg)

Tested on Windows:
FF Release **33.0**
Chrome Latest Release **38.0.2125.104 m**

Adding @wlach for review and @edmorley for visibility.