Simple DB backup/restore added

From the menu in the claims list tab, you can export the DB to a zip
file named 'yyyyMMddhhmmss-db-backup.zip' under '/sdcard/Open Tenure'.
Putting a file called 'opentenure.h2.db' under '/sdcard/Open Tenure'
causes the application to initializa its DB from it at start (if you
previously removed app data from Android settings). It is enough to
remove app data and expand a compressed backup uder '/sdcard/Open
Tenure' to go to a previous version of the DB.