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?

Is there a possibility to check if it fails? It seems to me that if I run `win_soft_link` without admin rights on Windows 7 it silently fails and does not throw an `OSError`. If there is no possibility to check this, the whole process of linking `conda` into the current environment and discarding the root environment's paths doesn't make much sense.