Hardcoded Microdata for com_content

This PR introduces Microdata tags to com_content in a hardcoded way.
This is based on some feedback that we don't need more parameters.

The PR is based on PR #3252 and the newer PR #3330. But unlike those two it doesn't use the Microdata library and instead just hardcodes the tags into the layouts.

#### Advantage
* Easy to understand
* Easy to change for endusers and designers who like to override a layout

#### Disadvantages
* You can't enable/disable Microdata tags globally or per article. You can of course use layout overrides and alternate layouts to achieve the same. It's also to note that there isn't really a reason to turn the Microdata off.
* You can't select a different Microdata type for an article using a parameter. It's hardcoded to be "Article" for single articles and "Blog" & "BlogPosting" for the blog layout in the category and featured view. You can of course use layout overrides and alternate layouts here as well to achieve the same, but you need to make sure yourself that the properties are valid for the selected type.

#### Testing
Test the different views of com_content and check the microdata output using the Google richsnippet test tool: http://www.google.com/webmasters/tools/richsnippets
If the vote plugin is enabled, the search result should show the voting as well.
The latest articles module contains also some Microdata tags with this change.