Avoid the costly shelling out to uname when on Windows.

As xolox#misc#os#is_mac() is invoked in some places without a previous conditional on xolox#misc#os#is_win(), avoid shelling out to uname for the Mac OS check when we can already cheaply determine via has() that we're on Windows.
Merge pull request #8: Avoid shelling out to uname on Windows

See pull request #8 on GitHub:
  https://github.com/xolox/vim-misc/pull/8