Modules and Calabash steps needed for dtbook-to-odt

The modules asciimath-utils and image-utils are needed by sbs:dtbook-to-odt. They are currently not used anywhere else, so if you don't want to put them in the assembly yet, that's okay, we can install them ourselfs. But the px:unzip step would be great so that we don't have to fork the assembly in order to create our own Debian package.

(Another solution would be to make config-calabash.xml a "real" Debian config file, which means Debian will do the right thing when updating. But I think it's more like a system file that should not be changed. Why do we need this file by the way, is it because we use the px: prefix instead of cx:?)