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?

You are right, I will try out making a version without any symlinking on Windows at all.