Return a Reference when loading a Bitmap

- This makes clear that pointer ownership is transfered to the caller.
  See see GotW 90 solution 2 at
  http://herbsutter.com/2013/05/30/gotw-90-solution-factories/
- Make AbstractBitmapStreamer::loadBitmap pure virtual
Make Serialization::loadBitmap return a Reference

This makes clear that pointer ownership is transfered to the caller.
See see GotW 90 solution 2 at
http://herbsutter.com/2013/05/30/gotw-90-solution-factories/
Make functions in BitmapUtils return a Reference

This makes clear that pointer ownership is transfered to the caller.
See see GotW 90 solution 2 at
http://herbsutter.com/2013/05/30/gotw-90-solution-factories/
Return a Reference from functions in font classes

This makes clear that pointer ownership is transfered to the caller.
See see GotW 90 solution 2 at
http://herbsutter.com/2013/05/30/gotw-90-solution-factories/
Make BitmapUtils a namespace
Accept a Bitmap as Reference in PixelAccessor

The PixelAccessor takes shared ownership of the Bitmap. Therefore,
passing a Reference by value is fine (see GotW 91 solution 3 (e),
http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/).
Bitmap as const ref in createSDLSurfaceFromBitmap

Notice: The SDL_surface is not correctly!
Accept a Bitmap as const ref in saveBitmap

Neither does the function take shared ownership nor does it change the
reference. Therefore, const ref is fine (see GotW 91 solution 3 (b),
http://herbsutter.com/2013/06/05/gotw-91-solution-smart-pointer-parameters/).
Code formatting