Set correct AppData dir for more Windows versions

Replace the hard-coded path with an environment variable supported by all versions of Windows since XP (`%APPDATA%` in Windows, `$APPDATA` to Vim).

By default, Windows XP stores Application Data in the folder:

    C:\Documents and Settings\<username>\Application Data

Windows Vista and later store AppData in:

    C:\Users\<username>\Application Data

or

    C:\Users\<username>\AppData\Roaming

You can, if you really need to, read all about the gory details of AppData here:
http://technet.microsoft.com/en-us/library/cc766489(WS.10).aspx