Split to Check|Backup and rest
throw subclass of Exception when backup failed
Reuse already downloaded package

Conflicts:
	lib/downloader.php
Show Retry button if check is failed
Handle server error
Further split: Check|Backup, Download, Replace
Remove legacy check method
Show path to the backup
Some visual improvements
Merge pull request #29 from owncloud/stable7-backport

Stable7 backport master improvements