[Pyallegro] Bitmap.from_string() support

[Grzegorz A. H. <gr...@ti...>]

Hi.

The attached patch provides to the Bitmap class a from_string() method
which is fit to work with external libraries like PIL. Such a "loose"
method can work with any kind of external lib without having to make
alpy aware of it. Along with this patch I'm attaching a small image
viewer I wrote which precisely exploits this from_string() method.

If somebody gives me CVS write access I will also slowly fill in the
python docstrings of the binding. These can be used to generate the
documentation or queried interactively with shells like iPython. And of
course I'll apply this from_string() patch as well.