ftpsync 20130501 performs AUIP check
Store per-subset serials in the db

Warn about regressions (switching back to an older serial) while at it,
related to issue #32
Don't print some diagnostic messages unless --verbose

The same information can be obtained directly from the database, so
it's not worth the noise.
Print a diagnostic message when overriding a subset's stamp
Check for oldstable-backports for backports mirrors

To reflect the merge of wheezy backports into the main archive.
Check for {Packages,Sources}.gz in security mirrors

The Release files are no longer updated and may go away any time soon.
Only attempt to store the serial if the 'serial' hash exists

For compatibility with old databases
Subsets age only needs to be checked once
Print the original stamp and the one we are going to use
Require at least amd64 and i386 on every subset

A bit of a work around for issue #33
Actually disable mirrors in subsets that doesn't meet the criterions

Refactor the code a bit while at it
Only apply the required archs criteria to 'archive' mirrors
Merge branch 'serial-regression'

Nothing is done about the issue, yet. Merging it only to already have a
record of such regressions.