We only build "stable" images now and push them first to the testing
branch and then to the stable branch. Thus, the versioning should always
be based on the newer of the two branches, stable and testing. A
paramter isn't needed anymore.
This closes#15 and #14.
This script takes the intended branch, "testing" or "stable", as the single
parameter. It then tries to autodetermine the correct release name, builds
gluon for the corresponding branch and all supported platforms (excluding
x86*), and optionally signs it if an ecdsutils keyfile is found (standard path:
~/.ecdsakey)