Use XDG Base Directories instead of storing in home directory

Use XDG Base Directories instead of storing in home directory

As defined in:
http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

This offers a number of advantages:
 * Allows sharing a home directory between different machines, or
sessions (eg. using NFS)
 * Cleanly separates cache, runtime (eg. sockets), or app data from
user settings
 * Supports performing smart or selective migration of settings
between different OS versions
 * Supports reseting settings without breaking things
 * Makes it possible to clear cache data to make room when the disk
is filling up
 * Allows us to write a robust and efficient backup solution
 * Allows an admin flexibility to change where data and settings are stored
 * Dramatically reduces the complexity and incoherence of the
system for administrators