Fail to clear contacts array on multiple backups

[drnida]

When ffosbr.contacts.backup() is called it loads all the contacts into an array in the contacts module. If backups is called again during the in the same instance of the app the contacts will be added to the array twice. The array should not persist beyond a single call to contacts.backup().

[thomasguerena]

@dcobbley and I fixed this bug while adding code for "onprogress" and "oncomplete" event handlers. Additionally, any calls to ffosbr.contacts.backup() while a contacts backup is in progress will immediately return without error.