Pre-Install official Guardian Project app repo to Repo-list

Please add the official Guardian Project app repo to the list of repostitores listed by default.

The URLs can be found at https://guardianproject.info/fdroid/

I would have submited a pull-request, but

1. I do no know where th get the pubkeys from
2. adding more repos as it is currently done in src/org/fdroid/fdroid/data/DBHelper.java leads to spagetti code. Is there a more elegant way?