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).
I don't think this is necessary, not even sure if it should compile.  [std::function's default constructor](http://en.cppreference.com/w/cpp/utility/functional/function/function) makes an 'empty function', which when checked with [operator bool](http://en.cppreference.com/w/cpp/utility/functional/function/operator_bool) evaluates to false.

So `hasDeleter()` above would be correct to just `{ return (bool)mDeleter; }`