Provide activate/deactivate functionality on Windows to match Unix (using batch files instead of symlinks)

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?

Mainly because of the different docstrings for Windows and Linux. If I move the `if platform == 'win': ... else: ...`  inside the `_write_entry_points` method, the docstrings mix up which might create more confusion than helping in understanding the necessity of the code.