imagen.image should not require PIL

[jbednar]

Currently, imagen.image can only be imported if PIL is available, yet it should be possible to use all of its features even without PIL because it can load Numpy .npy files, and Numpy is already a core dependency. It's slightly tricky to remove the dependency on PIL, because it's used in some parameter default values, but it's worth doing so that all functionality will be available without PIL. Plus it would then be easier to support non-PIL image loading libraries.