Method for purging notes

[atoponce]

When notes are purged, they should be securely scrubbed before deleting from disk. A triple pass of random data over the encrypted note should be more than sufficient, and because the notes will not likely be too large, this shouldn't tax the system too badly.

[atoponce]

Went with a single pass, rather than a triple pass. No assumptions are made about the underlying filysystem, thus taking into full account the possibility that scrubbing the note before deletion may actually overwrite existing encrypted data, leaving nothing behind.