Document diffy logo is CC-BY 3.0

Restructure the build process slightly so there is license information
attached to the diffy images.  The CC-BY 3.0 attribution information
and license will now appear in the documentation alongside all other
libraries used by Gerrit.

Change-Id: I8e3315fbe79ae1dc8e0444c6e6ed2ae241bcefb0
4 files changed