Amazon S3 support in runBackups.php, stage 1

* add --tmp option that tells the script to store backups in the
  system's tmp dir,
* prepend output filenames with Wikia's city_dbname
* do not create index.json, we no longer need it
add DumpsOnDemand::getPath() method.
fix a sprintf call in DumpsOnDemandCron.
Amazon S3 support in runBackups.php, stage 2

* added the --s3 option
* simplified other binary options
add DumpsOnDemand::putToAmazonS3() method.
Amazon S3 support in runBackups.php, stage 3

* added the actual logic for the --s3 option.
fix a shell command in DumpsOnDemand::putToAmazonS3()
serve dumps from Amazon S3 with Content-Disposition:attachment.
Special:Statistics: show timestamp of the latest dumps.
runBackups: delete the local copy once the file is put to S3
DumpsOnDemandCron: fix the shell command and database names