Kinder, gentler Python 3 compatibility

Looking at trying to minimize code changes for Python 3.

For now we are keeping byte str() in py2 and unicode str()
in py3. It's a pain to rewrite all string code to usee
u'' for py2 compatibility. However we may need to revisit
this for pexpect().

Created compatibility wrappers in util.py:

BaseString (replacement for basestring)
decode() (optionally decode utf-8 for py3)
encode() (optionally encode utf-8 for py3)

Also we've switched from the .iter*() flavors to their
static alternatives for py2 (note they are iterators in py3!)

Changed util.errRun and clean.sh to call decode()

bin/mn

@@ -256,7 +256,7 @@ class MininetRunner( object ):
         opts.add_option( '--arp', action='store_true',
                          default=False, help='set all-pairs ARP entries' )
         opts.add_option( '--verbosity', '-v', type='choice',
-                         choices=LEVELS.keys(), default = 'info',
+                         choices=list( LEVELS.keys() ), default = 'info',
                          help = '|'.join( LEVELS.keys() )  )
         opts.add_option( '--innamespace', action='store_true',
                          default=False, help='sw and ctrl in namespace?' )
@@ -286,7 +286,7 @@ class MininetRunner( object ):
                          metavar='server1,server2...',
                          help=( 'run on multiple servers (experimental!)' ) )
         opts.add_option( '--placement', type='choice',
-                         choices=PLACEMENT.keys(), default='block',
+                         choices=list( PLACEMENT.keys() ), default='block',
                          metavar='block|random',
                          help=( 'node placement for --cluster '
                                 '(experimental!) ' ) )