remove GD from configure.ac
Do not run mkimages.pl on bootstrap

* GD module is becoming tricky to install, and it is only used to
  generate the documentation little images

* Added images to repository, update images as necessary.
Add doc images to repository

* These little icons are used on the html documentation.
Track the images
cmake: do not search for Perl GD module
make: images are now created manually

* GD module is becoming a hassle to install.
update Readme
travis: simplify the build, drop GD
update changelog
Merge branch 'remove-GD'