Integrate with appveyor for Windows CI

Integrate with appveyor for Windows CI

appveyor.com is a hosted continuous integration service for Windows.
Add an appveyor.yml file containing build instructions.  The appveyor
virtual machines do not include the MFC libraries, so change
util/wshelper/resource.rc to avoid including <afxres.h> (which it does
not need) and add a build conditional for leash.

Right now we do not build the installers; the appveyor VMs do not
appear to have the version of the WiX toolkit we need, and we would
also have problems with the missing leash executable.