update to Jetpack 2.9.3

git-svn-id: https://plugins.svn.wordpress.org/jetpack-sharing/tags/2.9.3@892201 b8457f37-d9ea-0310-8a92-e5e31aec5664
add changelog line to readme

git-svn-id: https://plugins.svn.wordpress.org/jetpack-sharing/tags/2.9.3@892202 b8457f37-d9ea-0310-8a92-e5e31aec5664
Merge tag '2.9.3' of git://github.com/wp-plugins/jetpack-sharing into lechnology

Conflicts:
	readme.txt
	sharedaddy.php
	sharedaddy/sharing-service.php
	sharedaddy/sharing.php