Integrated old GPG replication patches

Integrates the following patches into the latest codebase (beta):

https://github.com/pecastro/musicbrainz-server/commit/f68c7575192a348dcd
c386404b7e56b5084a49ea

https://github.com/metabrainz/musicbrainz-server/compare/6a00461...mbh-1
53
Updated DBDefs declarations to use new style.

[CHG] Updated all "&DBDefs::"  to OO style refferences (DBDefs->).

[CHG] Commented out GPG_* options from the DBDefs.pm.sample to keep
inline with "Commented-out lines should generally have sane defaults"

[ADD] Added default values for all GPG_* keys into DBDefs/Default.pm.
By default, server will use no GPG signing/validation.
Updated code to use Try::Tiny
Updated signature process for replication packats and current database dumps

[CHG] Updated current database dump signatures to use detached
signatures, thus avoiding data duplication.

[CHG] Removed GPG_SIGNING_OPTIONS from the DBDefs keys in favour of the
existing --sign-with command line switch.

[CHG] Changed GPG binary name from /usr/bin/gpg to gpg, thus avoiding
"GPG not found" errors on systems where gpg is installed in
non-standard location (eg: /usr/local/bin/gpg)
Fixed a typo in a variable name
Refactored file fetch and signature verification code

[CHG] Moved all "use" statements to the top of the file

[CHG] Refactored file fetch and signature verification code to be as
close to the original as posible while implemmenting required features.

[ADD] Added a new DBDefs option which defines how validation deals with
the missing signature file.
Passthrough GPG exit code on FAIL
Updated arguments to the system command

Split "--option value" parameter for the systems command into two
separate arguments.
Merge branch 'master' into mbh-153
MBH-153: switch to using DBDefs for both encryption and signing keys
MBH-153: don't put the signature for the private dump on FTP
MBH-153: copy signatures to backup dir/ftp
MBH-153: update DBDefs.pm.sample
Merge remote-tracking branch 'ian/mbh-153'

* ian/mbh-153:
  MBH-153: update DBDefs.pm.sample
  MBH-153: copy signatures to backup dir/ftp
  MBH-153: don't put the signature for the private dump on FTP
  MBH-153: switch to using DBDefs for both encryption and signing keys
  Updated arguments to the system command
  Passthrough GPG exit code on FAIL
  Refactored file fetch and signature verification code
  Fixed a typo in a variable name
  Updated signature process for replication packats and current database dumps
  Updated code to use Try::Tiny
  Updated DBDefs declarations to use new style.
  Integrated old GPG replication patches