Add per-user settings support

* Add per-user settings support: just copy the `glances.conf` configuration file to `$XDG_CONFIG_HOME/glances` directory (Linux and *BSD) or `~/Library/Application Support/glances` (OS X). Per-user configuration override the default system-wide.
* Fix location of default configuration file: `/etc/glances` (Linux), `/usr/local/etc/glances` (*BSD and OS X)
* Bonus: Fix installation under virtualenv