Add support for copy on Windows systems

[SkylerLipthay]

I've added support for copying files and directories on Windows systems through the copy and xcopy DOS commands. I also fixed a bug for the success check in the Path.copy function by replacing success != 0 with v:shell_error != 0; I believe this fix represents the intended functionality.

My code has been tested on Windows XP SP3, Windows 8, and Mac OS X 10.6. Mac (Unix) functionality has remained unchanged.

[PhilRunninger]

@SkylerLipthay, I like this. It fills a hole in the Windows side of the code. Fix the merge conflicts, and I'll merge it in. Thanks for contributing.

[SkylerLipthay]

@PhilRunninger I've rebased from upstream and force pushed to my fork. I cannot test this functionality anymore, as I unfortunately don't have easy access to a Windows machine. Also, my Vimscript is pretty shoddy, so while everything looks good to me, I might be overlooking something obvious.

Side note: this was my first public OSS contribution, so I'm glad to see it get some closure! :) Thank you for your work.

[PhilRunninger]

I had to make a few minor changes to your PR, but overall it was good. Thanks for the update.

[symphony2014]

But it still does not work on my WIN10. It always remind me :

NERDTree: Could not copy node.

[PhilRunninger]

@symphony2014, please open a new issue with all relevant details filled in so that we can reproduce your issue.

Thanks.