Incremental backup retention system implemented 

I have implemented a backup retention system where backups are retained for varying levels of time.

By default:
*Daily backups are retained for 7 days
*Weekly backups are retained from Mondays for 4 weeks
*Monthly backups are retained from the first Monday of the month for 6 months

The retention lengths can be edited by editing variables in the .sh file

**Changes need to be implemented in the unix version**
**Remote backup deletion needs to be setup**

README updated with more documentation