implement og:image fallback image and og:description incl. featured layo...

Hi, seen a massive increase in reffered visits from facebook and G+ because of my previous pull request for the 'always filling of og:image'.
As it turns out there is a restriction: Facebook only accepts og:image images with minimum size of 200x200px. If the specified image is smaller, no image is taken or a random image is taken > this can and will lead to undesirable results.
1) What I have implemented in this pull request is to be able to specify a ogfb_image in the layout section of the template (back-end); here you can specify an image that should be used as fallback in case the automatically chosen image is to small.
In the code I read the size of the automatically chosen image; if to small (<200x200) then the ogfb_image will be used.

2) because the featured leayou was missing the og implementation and my website's home page is of type featured, I was missing out on all this great stuff :) So I have also implementated the og part in the featured layout.

3) while troubleshooting over at facebook it was complaining that my og:description was empty and although it could be filled with other data it shuld be fixed.... so I fixed it > it now holds the meta description (as this was the value that facebook was using itself.

Hope this helps :) to make this template even better then it already is :))))

O, although I have tested against several use cases and am using this in production, please review the code because I am not a developer :)