Improve Windows support and remove dead code

This is a follow-up of GH-18 based against current `master` now that `libvirt` has been merged.

It fixes GH-17, i.e. lets you use mccloud from a windows box, and includes the following:

 * add `::Mccloud::Util::Platform.windows?`
 * adapt rsync commandline so that it also works when using a Windows / Cygwin `rsync.exe`
 * remove dead code `lib/mccloud/util/rsync.rb` which adds a second rsync commandline shellout but is never used
 * remove dead code in `lib/mccloud/config.rb`