Add support for sync_folders action to support windows via WinRM

Prerequisites documented in readme file. I spent a bunch of time trying to create a repro case in the vagrantfile using a widely available windows image, but since I couldn't find any of those available with WinRM setup as required and the changes should be nondestructive to existing use cases, I'm going to file this as is.

There are a number of future improvements that I'd like to make but require work in other projects before they can be leveraged here. 
* Remove the requirement for basic auth and unencrypted traffic. Basic/unencrypted are fine for local VMs, but when you're talking to another machine over the network, we need more security. This may require a change in the WinRM gem and will require a change in the Vagrant core code. My ultimate goal is to get this talking securely over an Active Directory domain account.
* Change the folder syncing mechanism to use SMB, which appears to be the preferred / recommended sync method for Windows. For my use case, this will require implementing the SMB host support for linux and mac.