Added the ability to specify a custom deleter for your Texture...

...using the Format's ```setDeleter()``` method. If a custom deleter is specified, the texture's shared pointer will use it. This is a cleaner solution than the one I proposed earlier in #541 .

This raises the question whether or not we should also add this to Cinder's other Format classes (Fbo, Vbo, etc).
Thanks, didn't know that. Will make the adjustments.