PICARD-123: actually use default drive on Mac

This doesn't implement an automatic list of available drives on Mac,
but adds the libdiscid default drive to the list of drive candidates.

This should make disc lookups available on Mac without specifying
the drive (as /dev/rdisk*) in the settings.

Note that setting things like "1" or "2" as drives should also work
with a recent libdiscid. (>= 0.6.0).
use only one definition of get_cdrom_drives()
remove generic try-except

If we do want to catch generic unexpected tracebacks,
this shouldn't be done here.

Note that `import discid` might raise OSError
when the python module is found, but not libdiscid itself.
We might want to catch that explicitely,
but that is supposed to be prevented by whoever made `discid` available.
refactor cdrom code a bit
update NEWS for 123
Merge branch 'master' into always_default_drive

Conflicts:
	picard/util/cdrom.py
Merge branch 'master' into always_default_drive

Conflicts:
	NEWS.txt
Merge pull request #198 from JonnyJD/always_default_drive

PICARD-123: actually use default drive on Mac