Updated styling of comment history, towards #471.
Changed styling of attachment download icon to use background-image instead of an img inserted on the page.
Removed outdated screenshot.png because it doesn't appear to be used.

git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1467339 13f79535-47bb-0310-9956-ffa450edef68