Add support for artist aliases

Use MusicBrainz artist aliases to import files with artist names in an
appriorate locale
proceed past os.listdir errors in sorted_walk
ignore "System Volume Information" directory

This is a control directory on Windows whose permissions are always set very
restrictively:
http://blogs.msdn.com/b/oldnewthing/archive/2003/11/20/55764.aspx
fix crash when RG fields set to null

In general, we convert None values to type-based nulls in packed fields just
as we do for normal fields.
Merge pull request #102 from samdoshi/aliases

Add support for artist aliases (fixes GC-354)
refactor alias discovery into function (#102)
change option name to "languages"; docs (#102)
link to GitHub Issues
import.detail config option (GC-263)
add --flat option to import command (GC-202)
tag_album does not take timid as an argument

see SHA: 75d43270e881a7ecf16de4474b5fec54722ebf2e
go to next locale if no match in _preferred_alias

we should continue here rather than return so that we continue to search
for matches
add unit tests for aliases (#102)
Merge pull request #104 from samdoshi/initial_lookup_fix

tag_album does not take timid as an argument
changelog/thanks for #104
Merge pull request #105 from samdoshi/aliases

Unit tests and bug fix for aliases (#102)
convert: add missing syspath call (closes #106)
format None values as the empty string

Fix due to @pscn. Includes test. Closes #108.
fix bytes literals causing a Unidecode warning

This would trigger a warning in Unidecode when metadata was missing (which is
the only case when those empty-string literals are used). Closes #109, which
is a different fix for the same problem.
mbcollection: human-readable MB exceptions

Fixes #107 by wrapping the API invocation function with exception handlers.