Bug 1057349 - Differentiate and improve the logviewer icon

This work fixes Bugzilla bug [1057349](https://bugzilla.mozilla.org/show_bug.cgi?id=1057349).

In it we replace the existing logviewer icon `glyphicon-tasks` with the new Sheriff approved icon, to provide more differentiation between it and the adjacent raw log icon. Implied acknowledgment also by Sheriffs that the new icon is no longer localized or language independent.

I used the svg from Flaticon, and reversed and cropped it to maximize space.

Here's the before:

![logviewericoncurrent](https://cloud.githubusercontent.com/assets/3660661/5559337/28199120-8d17-11e4-8dd3-6bd53afeda58.jpg)

Here's the after:

![logviewericonproposed](https://cloud.githubusercontent.com/assets/3660661/5559340/2fd3ad6a-8d17-11e4-807f-04a6e08dbe74.jpg)

I spent a lot of time trying multiple sizes to try to reduce dither, and the best compromise appeared to be 14 by 16px (above). It's larger than the adjacent icons, but I imagine we can get away with it. I also tried larger sizes but all were a bit fuzzier (see below comparison of 14x16 vs. 15x17).

![logiconcomparisons_14x16vs15x17](https://cloud.githubusercontent.com/assets/3660661/5559346/7551af5e-8d17-11e4-8087-5c35fa1be9fe.jpg)

We needed to provide attribution, so I did that in the Help footer:

![attributionhelp](https://cloud.githubusercontent.com/assets/3660661/5559373/b1529cec-8d18-11e4-8934-79ff26507514.jpg)

Everything seems to behaving correctly on Firefox and Chrome. I've mimicked the hover highlight, via an opacity change on the svg.

Tested on Windows:
FF Release **34.0**
Chrome Latest Release **39.0.2171.95 m**

Adding @camd for review and @edmorley for visibility.