windows: leap.comon get_preffix call won't work in windows
current code depends on win32-something that's not pip-installable.
since I'm on it, and since this won't be used outside bitmask package, we should deprecate its use in leap.common. bring it to leap.bitmask.util
solution: get_leap_dir() function. use code from appdirs package, but do not depend on it.
(from redmine: created on 2016-10-06)