Properly encode localised mixer names before log

Mixer names in localized environment can contain non-ASCII characters. Handle them correctly before sending messages to logger.

How to reproduce:

    $ LC_MESSAGES=sk_SK.UTF-8 mopidy
    ...(snip)…
    INFO     Starting Mopidy core
    Traceback (most recent call last):
      File "/usr/lib/python2.7/logging/__init__.py", line 850, in emit
        msg = self.format(record)
      File "/usr/lib/python2.7/logging/__init__.py", line 723, in format
        return fmt.format(record)
      File "/usr/lib/python2.7/logging/__init__.py", line 464, in format
        record.message = record.getMessage()
      File "/usr/lib/python2.7/logging/__init__.py", line 328, in getMessage
        msg = msg % self.args
    UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 5: ordinal not in range(128)
    Logged from file actor.py, line 196
    INFO     Starting Mopidy frontends: MpdFrontend, HttpFrontend, MprisFrontend
    ...(snip)…

With fix you'll get:

    $ LC_MESSAGES=sk_SK.UTF-8 mopidy
    …
    INFO     Starting Mopidy core
    INFO     Audio mixer set to "alsamixer" using track "Hlavný"
    INFO     Starting Mopidy frontends: MpdFrontend, HttpFrontend, MprisFrontend