Add /etc configuration file for deamon installation, fixes stale README (#243)

fixes #242 

Implemented /etc/sendspin/settings-daemon.json configuration file, to
make Sendspin deamon configuration easier for admins and fix the
incorrect stale info in the README.

* Installer creates /etc/sendspin/settings-daemon.json during
installation
* Settings class reads from this file and merges fields with
~/.config/sendspin/settings-daemon.json, where fields in the latter take
precedence over /etc/sendspin/settings-daemon.json
* Backwards compatible as existing
~/.config/sendspin/settings-daemon.json still work
* Only runtime changes are saved to config, so user preferences are
respected.
 * README updated to reflect changes

Author: wildekek

README.md

@@ -106,7 +106,7 @@ pip install --upgrade sendspin
 sudo systemctl restart sendspin
 ```
 
-Your client name, audio device selection, and other settings in `/etc/default/sendspin` are preserved during the update.
+Your client name, audio device selection, and other settings in `~/.config/sendspin/settings-daemon.json` are preserved during the update.
 
 > **Note:** You do **not** need to uninstall and reinstall when updating. Your configuration (client name, audio device, delay settings) is stored separately and will be preserved.
 
@@ -395,7 +395,7 @@ sudo systemctl status sendspin   # Check status
 journalctl -u sendspin -f        # View logs
 ```
 
-**Configuration:** Edit `/etc/default/sendspin` to change client name, audio device, or delay settings.
+**Configuration:** Edit `~/.config/sendspin/settings-daemon.json` to change client name, audio device, or other settings.
 
 **Uninstall:**
 ```bash