Compressed PNG files a bit to reduce the size.
[FIXED JENKINS-17028]

Some browsers appear to cache 302 requests in violation of RFC
(I'm looking at you, Chrome: http://code.google.com/p/chromium/issues/detail?id=103458)

I also saw this behavior with Firefox, even though I couldn't locate any
bug report.

Sine Chrome alone is a big enough browser share, in this change I
modified the code to avoid 302 redirects and instead to service the
request with 200.

To avoid excessive data transfer, ETag is used to detect that the
browser has the image in cache.