Improve auth-token file path

On OSX store the authentication token under the ~/Library/gist/
directory [1]. On UNIX, follow the XDG Base Directory specifications [2]
and store the file in the directory specified by the XDG_CONFIG_HOME
environment variable, or if not set under ~/.config.

[1] https://developer.apple.com/library/mac/qa/qa1170/_index.html#//apple_ref/doc/uid/DTS10001702-CH1-SECTION5
[2] http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html