windows: Make appveyor.yml self-contained.
windows: Make appveyor.yml self-contained.

Add configuration which otherwise has to be set via the UI so the file is
more self-contained, and remove configuration which is not needed because
it's the same as the default.  The major change here is that for a while
now Appveyor has been using Visual Studio 2015 by default while we still
want to support 2013.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython