Update to latest v2.3.1 codebase from GitHub.

git-svn-id: https://plugins.svn.wordpress.org/scalable-vector-graphics-svg/trunk@949454 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update README to fix formatting.

git-svn-id: https://plugins.svn.wordpress.org/scalable-vector-graphics-svg/trunk@949456 b8457f37-d9ea-0310-8a92-e5e31aec5664