Merge branch 'import-tweaks-and-tests' into 'master'

Import tweaks and tests

This makes the code for `fdroid import` a bit more flexible and adds tests for the first time.  It also comments out options in `examples/config.py` that just mirror the defaults to make it clear that they are defaults, and help illustrate other options (this is standard procedure in default config files).

More details in the commit messages.

See merge request !76