Add file structure versioning
Add file structure versioning

This in the form of a number that can be used to discriminate what
version of wal-e was used to prepare the backup.  For example,
versions of wal-e previous to this may have stored the sentinel file
compressed as .lzo, or uncompressed.  Future versions may pack small
files, or not.  And so on.

Signed-off-by: Daniel Farina <3d0f3b9ddcacec30c4008c5e030e6c13a478cb4f@heroku.com>

equa/wal-e,ajmarks/wal-e,nagual13/wal-e,wal-e/wal-e,modulexcite/wal-e,alex/wal-e,RichardKnop/wal-e,tenstartups/wal-e,fdr/wal-e,ArtemZ/wal-e,fdr/heroku-wal-e,x86Labs/wal-e,intoximeters/wal-e,DataDog/wal-e,heroku/wal-e