#87 HOME not set correctly on Windows

[Lloyd Lim]

setup does not set the HOME environment variable
appropriately for Windows. On Unix, HOME is ~, which
allows each user to have his/her own set of Mash
preferences. On Windows, HOME is set to C:\, which
doesn't differentiate between users.

For a single user installation, it should be set to the
user's profile directory, which varies between
different versions of Windows.