Fix Python 3 encoding for error messages and `VERBOSE` output

The extractor always returns byte strings, which is fine for Python 2.
Python 3 `print()` expects unicode strings, rather than byte strings. So if `str == byte` (Python 2) we do nothing. Otherwise we decode the msg if it isn't already a unicode string.

There was a different problem when `VERBOSE` was activated. An the byte string coming from the extractor was encoded again (which is fine as long as it is ascii on Python 2), but as above, using `print()` with a byte string on Python 3 is wrong.

The result on Python 3 with errors without this fix is this:
```
[:( unk 5  ] /var/data/music/mp3-db/In Strict Confidence/2002: Mistrust the Angels/09 - In Strict Confidence - It Seems Lost....mp3
b'Process step: Read metadata\nProcess step: Compute md5 audio hash and codec\n\x1b[0;33m[ WARNING  ] \x1b[0mAudioLoader: invalid frame, skipping it: Invalid data found when processing input\nProcess step: Replay gain\n\x1b[0;33m[ WARNING  ] \x1b[0mAudioLoader: invalid frame, skipping it: Invalid data found when processing input\n\x1b[0;33m[ WARNING  ] \x1b[0mAudioLoader: invalid frame, skipping it: Invalid data found when processing input\nERROR: File looks like a completely silent file... Aborting...\n'
```

I also added a `compat.encode()`, which was used until I found out even the previous `encode()` was misplaced (see above). I kept it in the code unused. It might help out later on.

Note that I have "utf-8" and "ascii" as encodings in `compat.py` since that was in the code I was replacing.
This should probably replaced with `sys.stdout/stdin.encoding`. I left in comments about that.

Getting Unicode output to work on Windows is a different beast.
For the problem and the solution have a look in JonnyJD/isrcsubmit#40 and/or 8f931940ec9b45e2ee72db0bc19e97a9f25bf6f2.
Short: You have to make a wrapper to change the codepage to cp65001 and tell Python that this is "~ utf-8".