Merge branch 'fdroid-build-in-git-repo' into 'master'

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

See merge request !62