Merge branch 'bsd-osx-porting-and-tests' into 'master'

BSD/OSX porting and tests

This makes `fdroid init` and `fdroid update` work on BSD/OSX and maybe Cygwin also (I didn't test Cygwin).  It also makes the install process more tolerant, like not requiring `python-magic` to run, while using it if its available.

Some fixes and additional tests are also included.  The commit messages have more info.

See merge request !56