JMicrodata update

— Updated the types.json file with the new available Types and Properties
— Small documentation & code refactoring
— Bug fix: if specified, return an empty string on fallback or disabled library, even if a content is available https://github.com/joomla/joomla-cms/pull/4108/files#diff-cd3328236e7aecb09373c1c564b9bbb8R256
Is the intent to eventually remove this method in favor of using htmlTag directly? If so, since it is a pubic method, I'd suggest marking it as deprecated for removal in Joomla 4. 

If not, 
![324636i89043c7cb36e04ca](https://cloud.githubusercontent.com/assets/634119/3934218/d022fcf0-2488-11e4-8c5c-4bf721639690.gif)