WinRM synced folder plugin

This adds (very low-priority) support for WinRM synced folders. The real work was done by @sneal to implement the WinRM FileManager. I just exposed it as a plugin.

It's probably the slowest synced folder plugin, but it has the advantage of working directly over the WinRM communicator. So if the communicator is working and secure so is this synced folder plugin - there are no other provider, platform or capability limitations.

Currently this will never be selected as the default synced folder type, unless you are are a host where all other types are unusable. In particular, if "rsync" is installed on the host it will always win over WinRM, both because the rsync usability check only takes the host into account, and because there are capability detection bugs for Windows guests that result in false positives.

So, Vagrant will sometimes pick a non-functional rsync or other synced folder implementation over a working WinRM synced folders. Right now the only way to use this is to explicitly set a specific synced folder's type to winrm.