some fixes to help port to Windows

This is some work towards porting fdroidserver to windows. It includes test runs on CI providers:

• https://travis-ci.org/fdroidtravis/fdroidserver/builds/
• https://ci.appveyor.com/project/fdroid/fdroidserver

I welcome help finishing this, especially since I do not have any Windows boxes of my own! So I have no idea whether this should target Cygwin, MSYS, Powershell, Chocolatey, etc.

This will get a lot easier once we remove support for using the Android SDK build tools and jarsigner -verify #94 since there are a bunch of path and newline errors related to running processes in Python.