Store the token config file in $XDG_CONFIG_HOME

This moves the `~/.jist` file into the `$XDG_CONFIG_HOME` directory as to follow the [Freedesktop.org XDG base directory specifications](http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html).

This will read the token from the `$XDG_CONFIG_HOME/jist` file, or if the `$XDG_CONFIG_HOME` environment variable isn't set, it will default to `~/.config/jist`.

I haven't really considered the implications for OSX or Windows for this patch. I did [a little googling](http://stackoverflow.com/questions/3373948/equivalents-of-xdg-config-home-and-xdg-data-home-on-mac-os-x) on what the equivilents are on OSX for the XDG folders, but didn't come to any solid conclusions. I didn't do any looking into if it makes sense for it to be stored in `~/.config/` on windows (TBH, I really have no idea what that would even expand to on windows).

Also, users with a `.jist` file currently in their home directory will have to move it to `~/.config/jist`.