run `fdroid build` straight out of an app's git repo

This creates a new metadata file type that is meant to be included in the git repo of the app to be built. It uses the same formats as metadata/, e.g. .txt, JSON, XML, YAML. The filename is instead .fdroid.(json|txt|xml|yml). This metadata then lets the user run fdroid build directly in the git repo of the app, and it will run the build as any other fdroid build.

@mvdan @CiaranG @krt @pserwylo @NicoAlt @parmegv feedback, flames, comments wanted

Given the very raw state of testing Android apps with gitlab-ci, I think this is a great opportunity for fdroidserver to become the standard method for testing Android apps with gitlab-ci, starting with this merge request to provide fdroid metadata embedded in the project. What still needs to added is something like fdroid builddepends debian and fdroid builddepends android which reads the metadata and dumps out a list to be fed to apt-get install and android update sdk --no-ui --filter respectively, so that a .gitlab-ci.yml can look like:

apt-get -y install fdroidserver
apt-get -y install `fdroid builddepends debian`
echo y | android update sdk --no-ui --all --filter `fdroid builddepends android`
fdroid build

Then that would become a template .gitlab-ci.yml that should work on most Android apps.

This was marked Work-In-Progress because it depends on !57 (merged)