Add functions for listing cards and their names

The cards() method does not guarantee that the index in its return
value is the same as the actual card index. Provide a way to get this
information in the form of a card_indexes() function, returning a
list of available card indexes.

Add another method, card_name(), which, given a card index, returns
the short and long names of that card.

mixertest.py

@@ -23,6 +23,12 @@ import sys
 import getopt
 import alsaaudio
 
+def list_cards():
+    print("Available sound cards:")
+    for i in alsaaudio.card_indexes():
+        (name, longname) = alsaaudio.card_name(i)
+        print("  %d: %s (%s)" % (i, name, longname))
+
 def list_mixers(kwargs):
     print("Available mixer controls:")
     for m in alsaaudio.mixers(**kwargs):
@@ -113,6 +119,8 @@ if __name__ == '__main__':
         else:
             usage()
 
+    list_cards()
+
     if not len(args):
         list_mixers(kwargs)
     elif len(args) == 1: