[tmux] Make the output string from mpc decode correctly

When mpd is playing a song with a non-ascii name, `now_playing.split('\n')` will throw the following exception:

    2014-05-26 08:16:55,651:ERROR:tmux:now_playing:Exception while computing segment: 'ascii' codec can't decode byte 0xc3 in position 57: ordinal not in range(128)
    Traceback (most recent call last):
    File "/home/ymf/.local/lib64/python2.7/site-packages/powerline/theme.py", line 75, in get_segments
     contents = segment['contents_func'](self.pl, segment_info)
        File "/home/ymf/.local/lib64/python2.7/site-packages/powerline/segment.py", line 96, in <lambda>
     contents_func = lambda pl, segment_info: _contents_func(pl=pl, **args)
        File "/home/ymf/.local/lib64/python2.7/site-packages/powerline/segments/common.py", line 917, in __call__
     func_stats = player_func(**kwargs)
     File "/home/ymf/.local/lib64/python2.7/site-packages/powerline/segments/common.py", line 982, in player_mpd
     now_playing = now_playing.split('\n')
     UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 57: ordinal not in range(128)

I guess this is related to locale settings, however, when I copy out the problematic code segment and test it separately in another python file, the error won't come up. Nevertheless, according to http://stackoverflow.com/questions/22267629/unicodedecodeerror-ascii-codec-cant-decode-byte-0xea-in-position-8-ordinal, here is a quick fix.