Fix broken HTML; `<legend>` should be closed.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@867165 b8457f37-d9ea-0310-8a92-e5e31aec5664
Implement auto-tweet control when editing posts. Version bump to 0.7.3.

git-svn-id: https://plugins.svn.wordpress.org/tumblr-crosspostr/trunk@867167 b8457f37-d9ea-0310-8a92-e5e31aec5664