Closed Issue: Add Separate and Combine methods. [1011]

Fix the _Seperate_ method in MagickImage and add a _Combine_ method to MagickImageCollection.
Comments: Resolved with changeset 27163: MagickImage.Separate no longer modifies the image, it returns an IEnumerable.
Added some methods to MagickImageCollection.
LayerMethod enum is no longer public.
Removed LayerMethod argument from MagickImageCollection.Merge.