Add winrt support

This PR builds on https://github.com/mattn/go-ole/pull/44 and implements basic support for winrt.  Most of the file changes are coming from that PR's commit.  This PR only adds https://github.com/ziuchkovski/go-ole/commit/b460b1167b34af3992b6def70fc50c9ff655dd2f on top.

I'm using this code to display windows 8-style toast notifications in a separate (unpublished and unpolished) project.  It's in "works for me" status.  :)