Update `oauth_client_class` to latest version.

git-svn-id: https://plugins.svn.wordpress.org/wp-crosspost/trunk@998139 b8457f37-d9ea-0310-8a92-e5e31aec5664
Import major features from Tumblr Crosspostr, version bump to 0.3.

git-svn-id: https://plugins.svn.wordpress.org/wp-crosspost/trunk@998142 b8457f37-d9ea-0310-8a92-e5e31aec5664