Package qpress in Percona repositories

What is still missing with qpress-compressed backups is an automatic way to uncompressed a backup. Adding an --uncompress option to innobackupex would be rather easy. However, since qpress is not even available in most distros, we have to package it, make it available in Percona repositories and make XtraBackup packages depend on the qpress package.

Update 15/07/2013: it's been decided to skip package dependency for now. We'll just document that qpress can be installed from Percona package repositories.