Reduce image size.

Checking the site with the PageSpeed extension, one of the reports was
in reducing image size.  This reduced the number of bytes by 67%.

It was done with optipng -o7 -fix *.png in the app/assets/images dir.

References:
  optipng: http://optipng.sourceforge.net/
  PageSpeed extension: https://developers.google.com/speed/docs/insights/using_chrome
Merge branch 'master' into logo-white-compress

Conflicts:
	app/assets/images/logo-black.png
	app/assets/images/logo-white.png
Another optipng run over png's.

Ran optipng -o7 -fix over all png files. This should make pages load a
tiny bit faster (you can test this with the pagespeed tool on firefox or
chrome).

It might be an idea to add *lossless* image compression optimization
runs to the release process.
Also optimise gifs with gifsicle.

These are the same gif files just after running
gifsicle -b --optimize **/*.gif
Merge pull request #5476 from lyda/logo-white-compress

Reduce image size.