Detect and handle CD-R discs (#154)

* Use cdrdao to detect CD-Rs and continue ripping only if the argument --cdr is passed.

* Get and display disc format from MusicBrainz.

* Add info about CD-R detection to logger

* Remove Musicbrainz medium format info

* add cdrdao command to logger

* Fix for PEP8

* Fix line break

whipper/program/cdrdao.py

@@ -47,6 +47,19 @@ def read_toc(device, fast_toc=False):
     return toc
 
 
+def DetectCdr(device):
+    """
+    Return whether cdrdao detects a CD-R for 'device'.
+    """
+    cmd = [CDRDAO, 'disk-info', '-v1', '--device', device]
+    logger.debug("executing %r", cmd)
+    p = Popen(cmd, stdout=PIPE, stderr=PIPE)
+    if 'CD-R medium          : n/a' in p.stdout.read():
+        return False
+    else:
+        return True
+
+
 def version():
     """
     Return cdrdao version as a string.