Include images from markdown folders

Make images from md folders available, so a post or page can be one unit in a folder.

With the patch, I can put all my pictures into the post/page folder:
md/diving/2014-12-05-egypt.md
md/diving/turtle.jpg
md/diving/trex.jpg

The patch is very basic and does not handle conflicts between images with the same name and different content. Anyway this is just an issue if such functionality is used. It is not changing the possibility of having all stuff at one place.