Allow users to set remote images on all cell types

Hey @clayallsopp, I saw [this discussion](https://groups.google.com/forum/#!topic/rubymotion/HNWxKQOp0K8) on the rubymotion google group and figured i'd start work to allow this to happen easily.

I've only worked on this for a few minutes, but figured i'd start the PR and get buy-in form you before I go and write specs and extend it any further.

Let me know if you like this and if I should flesh out the specs for it.

-Mark