add Windows Phone 8.1 MSVC2013 to CI

As all release configurations should be CI-tested, Windows Phone should be added to CI. It only needs to be build-tested (not auto-tested, yet). The platform occasionally breaks due to headers not present in the Windows Phone SDK getting included; build testing would solve this.

The platform requires MSVC2013 Update 2 or 3. The testconfig can be cloned from the existing winrt-x64-msvc2013 testconfig.

There are two mkspecs to choose from, winphone-x86-msvc2013 and winphone-arm-msvc2013. As the ARM version is more "exotic" and will be the one used in production, it is probably the right one to go with.