* Added separate Disk().check_attributes() function
* Can be used to check all KNOWN_ATTRIBUTES or just blocking ones
* Renamed ATTRIBUTES to KNOWN_ATTRIBUTES for clarity
* Renamed 'Critical' column to 'Blocking'
* Added '(Failed)' note to attribute report
* Addresses issue #131
* Raises an exception for blocking events
* Removed "Ignore" column from ATTRIBUTES
* Listed attributes should either be warnings or errors
* Only 'Critical' attributes should block futher tests
