fix windows problem with bin path (backslashes)

RE: https://github.com/mikeerickson/gulp-phpunit/issues/4 and https://github.com/mikeerickson/gulp-phpunit/issues/5

Fixes problem where windows requires double backslashes in path to phpunit.

Fixed for default path and added note for windows users if using other than the default path.

From what I can tell using os.platform returns win32 starting with XP up to Windows 7 regardless if it is 32 or 64 bit