Swap the order of `zip` and `ZipArchive` for file manifest backups

On large sites, the restart time for `ZipArchive` backups is quite horrendous. `zip` is still the most performant.
Make sure the parent's `__construct()` is called when this subclass is inited

We want to get the `set_time_limit()` and all that goody stuff
Give backup heartbeat requests a bit of time to actually restart
Glob all files in backups dir for filemtimes
Merge pull request #142 from humanmade/glob-all-files

Glob all files in backups dir for filemtimes
Update readme for #141
Merge pull request #141 from humanmade/backup-method-order-141

Improvements to backup restarting