implement og:image fallback image

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.
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.
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 :)