Remove border from iFrames in Firefox

Unfortunately, Firefox doesn't seem to respect any CSS settings to
control margin/border/padding in iFrames. Without setting marginwidth=0,
a ~5px is visible at the top and left of iframes.
Use black as background color for iframes

Given the vast majority of iframes being used are graphite graphs, use
black as background color. As a future improvement, set a graphite
related CSS class for graphite graphs and only use black there.
add graphite_frame class to graphite frames
Merge pull request #98 from prometheus/iframe-firefox-styling

Iframe firefox styling