Jetpack: Tagging 2.4.2b1 beta.

git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@767005 b8457f37-d9ea-0310-8a92-e5e31aec5664
Jetpack: Updating trunk readme to match branches/2.4 with pending changes in 2.4.2

git-svn-id: https://plugins.svn.wordpress.org/jetpack/trunk@767006 b8457f37-d9ea-0310-8a92-e5e31aec5664