Dispose of four sub-images in stitchImages methods as this was causing an
exception to be thrown when GDI ran out of resources. Also return Bitmap
from both stitchImages and readImage.
Added ImageReader and TileReader classes to move image and tile reading
functionality outside of PyramidGenerators. Also added ImageUtils class
that contains functionality to combine four subimages into one. Updated
AsynchronousPyramidGenerator to use these new classes to work an a real
image. PyramidGenerators continue to ignore Stream argument to the generate
method.
Updated SequentialRecursivePyramidGenerator to use TileReader and
ImageUtils class.  This PyramidGenerator throws an OutOfMemoryException no
matter how many levels are used - even just two levels.  Don't know why.