Provide activate/deactivate functionality on Windows to match Unix (using batch files on XP)

Hi, as a follow-up to https://github.com/conda/conda/pull/885 I have added the discussed poor-man-symlink (=batch file) implementation to  oddodood's scripts. @asmeurer: Is this like you thought it should be?

I guess if it fails silently check if the file that was supposed to be created exists.