Fix warnings using Win32 Unicode functions instead of ANSI functions in curl lib
Avoid calling Image::getPixelAddress() (virtual function) in several ImageImpl functions
Specialize ImageImpl::clear for Indexed and Bitmap images
Add ImageBuffer class

It tries to mitigate issue #239, but it's not a full fix yet. A good
possibility would be to change the internal structure of images, creating
tiles. So we don't need to allocate/copy/clear/etc. huge images in each
step.
Improve put/get_pixel_fast() functions to avoid virtual function call
Update TODO.md
Improve creation of Dirty() instance (fix issue #239)

Now Dirty() ctor receives the bounds, so we don't need to iterate over
the whole image to find/shrink the modified region.