Add "Post to Twitter" permission

The "post to Twitter" sub-module is really not usable because there is no permission introduced to be able to limit this to specific roles.

If you set it to allow posting to Twitter for node type "Blog", all users that have the ability to create Blogs will have the option to post to Twitter. This is not good, you need an additional permission on the Twitter module to let us choose which roles can post their blogs to Twitter.