gc and encrypted metafiles

[Stéphane Alnet]

>> 1. Moving the decryption code to a shared location and calling it from both
>> places (your proposed solution).

I did some work tonight on this. The first draft can be seen at
  http://stephane.shimaore.net/code/style.diff
(diff against svn 220)

I'm mostly looking for style feedback since I haven't tested a single
line yet. I created two new files, Brackup::Decrypt and
Brackup::DecryptedFile. Brackup::Decrypt contains all the decryption
code for both in-memory datarefs and files; DecryptedFile is an
unlink-on-destroy file object.

In Brackup::Restore I removed the decryption code and adapted a few
locations to match the new patterns. In Brackup::Target I added the
decryption code to gc().

Stéphane