Make required module GD optional

Boris,

This pull request is to make the GD module optional when installing this package. I think this will help users of EBook::MOBI who don't have the GD library installed and do not require image manipulation.

This builds and tests OK with and without the GD library installed but I have not been able to test its impact on EBook::MOBI::Image.

Let me know if you have any comments or want any changes to the p.r.

Thanks
Rupert