Improve duplicity helper

* Read a config specific to its installed name, so we can use
  same script for different (eg hourly, daily) sets
* Handle 'recover' option better
* Only cover a limited set of the duplicity commands -- recovery focussed.
* Good usage output
* README
* CHANGELOG
Fix for Ubuntu 14.04
Reduce hourly backups to 6 days; allow override

Make sure we can override the default number of days that we
keep backups for both daily and hourly.

It is however impractical to store huge number of days of hourly
backups (due to the number of files to sync with at the remote). An
hourly backups usefulness deminishes over time, so only store 6 days
by default.

*** It is vital that the backup area covered by the hourly backup is
    also covered by the daily backup for this logic to make sense ***
Add backupa and recovery details to README
README example wording correction
RST error correction
Merge pull request #1 from ministryofjustice/helper_update

Duplicity recovery helpers; documentation