convert: Character encoding/path issue on Windows

The convert plugin seems to have issues with certain characters. Here is the cmd.exe output when importing:

```
C:\Users\Michael\Desktop\blink-182\blink-182>beet import .

C:\Users\Michael\Desktop\blinkΓÇÉ182\blinkΓÇÉ182 (14 items)
Tagging:
blinkΓÇÉ182 - blinkΓÇÉ182
URL:
http://musicbrainz.org/release/f0359d59-315e-45e6-bf7b-170754bac198
(Similarity: 100.0%) (CD, 2003, US, Geffen Records)
```

Everything works fine here but notice how the `-` becomes `ΓÇÉ`. Then when I try to convert one of the tracks:

```
C:\Users\Michael>beet -v convert -d . stockholm
user configuration: C:\Users\Michael\AppData\Roaming\beets\config.yaml
data directory: C:\Users\Michael\AppData\Roaming\beets
Sending event: pluginload
library database: C:\Users\Michael\Music\Library.blb
library directory: C:\Users\Michael\Music\Files
Sending event: library_opened
blinkΓÇÉ182 - blinkΓÇÉ182 - Stockholm Syndrome
Convert? (Y/n) y
Encoding C:\Users\Michael\Music\Files\blinkΓÇÉ182\blinkΓÇÉ182\05 Stockholm Syndrome.flac
Encoding C:\Users\Michael\Music\Files\blinkΓÇÉ182\blinkΓÇÉ182\05 Stockholm Syndrome.flac failed. Cleaning up...
Command ffmpeg -i C:\Users\Michael\Music\Files\blinkΓÇÉ182\blinkΓÇÉ182\05 Stockholm Syndrome.flac -y -vn -aq 2 C:\Users\Michael\blinkΓÇÉ182\blinkΓÇÉ182\05 Stock
holm Syndrome.mp3 exited with status 1
ffmpeg version 2.2.3 Copyright (c) 2000-2014 the FFmpeg developers
built on Jun 19 2014 20:36:23 with gcc 4.8.3 (GCC)
configuration: --disable-static --enable-shared --enable-gpl --enable-version3 --disable-w32threads --enable-avisynth --enable-bzlib --enable-fontconfig --ena
ble-frei0r --enable-gnutls --enable-iconv --enable-libass --enable-libbluray --enable-libcaca --enable-libfreetype --enable-libgme --enable-libgsm --enable-libi
lbc --enable-libmodplug --enable-libmp3lame --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libopus --enable-librtmp --enabl
e-libschroedinger --enable-libsoxr --enable-libspeex --enable-libtheora --enable-libtwolame --enable-libvidstab --enable-libvo-aacenc --enable-libvo-amrwbenc --
enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxavs --enable-libxvid --enable-decklink --en
able-zlib
libavutil 52. 66.100 / 52. 66.100
libavcodec 55. 52.102 / 55. 52.102
libavformat 55. 33.100 / 55. 33.100
libavdevice 55. 10.100 / 55. 10.100
libavfilter 4. 2.100 / 4. 2.100
libswscale 2. 5.102 / 2. 5.102
libswresample 0. 18.100 / 0. 18.100
libpostproc 52. 3.100 / 52. 3.100
C:\Users\Michael\Music\Files\blink‐182\blink‐182\05 Stockholm Syndrome.flac: No such file or directory
Conversion failed!

Sending event: cli_exit
```

Notice how the `-` is `‐` in the ffmpeg command but `‐` in the output. This makes me think it has something to do with character encoding and `subprocess.popen`.

P.S. I only modified the source to display the ffmpeg output. Here's the `git diff`:

```
diff --git a/beets/util/__init__.py b/beets/util/__init__.py
index 38cecd7..0f8285c 100644
--- a/beets/util/__init__.py
+++ b/beets/util/__init__.py
@@ -665,6 +665,7 @@ def command_output(cmd, shell=False):
raise subprocess.CalledProcessError(
returncode=proc.returncode,
cmd=' '.join(cmd),
+ output=stderr
)
return stdout

diff --git a/beetsplug/convert.py b/beetsplug/convert.py
index 0b87fb7..73a4664 100644
--- a/beetsplug/convert.py
+++ b/beetsplug/convert.py
@@ -116,6 +116,7 @@ def encode(command, source, dest, pretend=False):
exc.cmd.decode('utf8', 'ignore'),
exc.returncode,
))
+ log.debug(u'{0}'.format(exc.output.decode('utf-8')))
util.remove(dest)
util.prune_dirs(os.path.dirname(dest))
raise
```