Image primitives

[fryguybob]

(Imported from http://code.google.com/p/diagrams/issues/detail?id=22. Original issue from byor...@gmail.com on April 18, 2011, 09:37:33 PM UTC)

The 2D section of the standard library should have an image primitive, for including external images in a diagram. The design of how exactly this should work is open to debate. Will also need to add support for it to the cairo backend.