Add IFTTT notification service

Add the ability to trigger a custom event on the Maker channel provided
by IFTTT service. With this trigger one can program actions on another
channels. Examples:

-Log backup notification to a google spreadsheet;
-Receive notification on devices with IFTTT application installed;
-Send e-mails
-Log to evernote
-Turn a Philips Hue light green for good backup or red on errors

IFTTT Maker channel accepts three values (named value1, value2, value3)
as part of the trigger. This implementation sends the content of
NOTIFICATION_CONTENT as "value1" parameter and let "value2" be defined at
configuration file. At this time "value3" is not used.

Signed-off-by: Josenivaldo Benito Jr <jrbenito@benito.qsl.br>

duplicity-backup.sh

@@ -416,6 +416,9 @@ send_notification()
     if [ "${NOTIFICATION_SERVICE}" = "slack" ]; then
       curl -X POST -H 'Content-type: application/json' --data "{\"text\": \"${NOTIFICATION_CONTENT}\", \"channel\": \"${SLACK_CHANNEL}\", \"username\": \"${SLACK_USERNAME}\", \"icon_emoji\": \":${SLACK_EMOJI}:\"}" "${SLACK_HOOK_URL}"
       echo -e "Slack notification sent to channel ${SLACK_CHANNEL}" >> "${LOGFILE}"
+    elif [ "${NOTIFICATION_SERVICE}" = "ifttt" ]; then
+    	curl -X POST -H 'Content-type: application/json' --data "{\"value1\": \"${NOTIFICATION_CONTENT}\", \"value2\": \"${IFTTT_VALUE2}\"}" "${IFTTT_HOOK_URL}"
+      echo -e "IFTTT notification sent to Maker channel event ${IFTTT_EVENT}" >> "${LOGFILE}"	
     elif [ "${NOTIFICATION_SERVICE}" = "pushover" ]; then
       curl -s \
       -F "token=${PUSHOVER_TOKEN}" \