Refresh the UI with a Treeherder color scheme (#111)

This addresses issue #111, updating the legacy Netscape color theme to one which will match Treeherder going forward, [as mentioned by Dave](https://github.com/mozilla/mozmill-dashboard/issues/111#issuecomment-45846780).

The total changes, including some minor UI tweaks to make it look cleaner, are:

* removal of unused menu png's - replaced by solid color matching Treeherder
* 1px black divider lines added to the menu block matching Treeherder
* white main-menu hover text color for more perceptible hover
* solid color panel removal from the Report Information table
* bold text removal from Report Information table
* 5px table padding removal from Results tables
* bold removal from row1 Results tables
* half-luminance 'unselected' sort-icon for row1 Results tables

I had to add a discrete css selector for the Report Information table to re-add the 5px padding removed during the overall table padding removal. Otherwise that table looked too compressed against its boundary.

Adding @whimboo for visibility.
All links in the header are actually part of the `.topnav` navigation. We should remove this selector completely.