Display link to published, crossposted posts as an admin notice.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@867817 b8457f37-d9ea-0310-8a92-e5e31aec5664
Optionally use post excerpt rather than body as crosspost content.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@867818 b8457f37-d9ea-0310-8a92-e5e31aec5664
Update readme and screenshots, `.pot` file, version bump to 0.7.4.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@867819 b8457f37-d9ea-0310-8a92-e5e31aec5664