Switch to cwRsync and patch Vagrant to make rsync-based synced_folders work (#91)

[tknerr]

This PR remove the included cygwin-ssh and cygwin-rsync files in favor of cwRsync (free edition) and fixes #91:

• this makes it work on Windows 8.1
• plus we don't have to maintain the ssh / rsync binaries ourselves
• plus the cwRsync-included ssh.exe reacts much faster

Also, instead of modifying the VAGRANT_DETECTED_OS environment we patch vagrant (which is the alternative suggestion for working around hashicorp/vagrant#4073):

• does not require to put cygdrive.exe on the PATH
• does not bear the danger of other side-effects due to changing the VAGRANT_DETECTED_OS
• is faster because using cygdrive.exe was really slow ;-)