New feature: export wallet to incognito format

  Incognito wallet is 48, 56 or 64 bytes of apparently random data.

  Allows user to hide wallet data in a pre-existing file or on a disk
  partition (preferably filled in advance with random data).

  Can be used to hide wallet securely in unencrypted cloud storage or
  on paper, without revealing the nature of the data.

  Data may be written at a user-specified offset into the file or
  partition, in which case user must remember the offset.

  Each export operation uses a new random init vector to create different
  data each time.  This allows the user to hide wallets at different
  locations on the Net without detection.

  User must remember hash preset in addition to passphrase (though trial
  and error can be used if it's forgotten).

  Fully integrated with address generation and tx signing operations.
Version 0.7.1