Initial python3 changes

Mostly just updating libraries, removing string encoding/decoding,
fixing some edge cases. No new functionality was added in this
commit.

lib/beetsplug/rewrite.py

@@ -0,0 +1,73 @@
+# This file is part of beets.
+# Copyright 2016, Adrian Sampson.
+#
+# Permission is hereby granted, free of charge, to any person obtaining
+# a copy of this software and associated documentation files (the
+# "Software"), to deal in the Software without restriction, including
+# without limitation the rights to use, copy, modify, merge, publish,
+# distribute, sublicense, and/or sell copies of the Software, and to
+# permit persons to whom the Software is furnished to do so, subject to
+# the following conditions:
+#
+# The above copyright notice and this permission notice shall be
+# included in all copies or substantial portions of the Software.
+
+"""Uses user-specified rewriting rules to canonicalize names for path
+formats.
+"""
+
+import re
+from collections import defaultdict
+
+from beets.plugins import BeetsPlugin
+from beets import ui
+from beets import library
+
+
+def rewriter(field, rules):
+    """Create a template field function that rewrites the given field
+    with the given rewriting rules. ``rules`` must be a list of
+    (pattern, replacement) pairs.
+    """
+    def fieldfunc(item):
+        value = item._values_fixed[field]
+        for pattern, replacement in rules:
+            if pattern.match(value.lower()):
+                # Rewrite activated.
+                return replacement
+        # Not activated; return original value.
+        return value
+    return fieldfunc
+
+
+class RewritePlugin(BeetsPlugin):
+    def __init__(self):
+        super().__init__()
+
+        self.config.add({})
+
+        # Gather all the rewrite rules for each field.
+        rules = defaultdict(list)
+        for key, view in self.config.items():
+            value = view.as_str()
+            try:
+                fieldname, pattern = key.split(None, 1)
+            except ValueError:
+                raise ui.UserError("invalid rewrite specification")
+            if fieldname not in library.Item._fields:
+                raise ui.UserError("invalid field name (%s) in rewriter" %
+                                   fieldname)
+            self._log.debug('adding template field {0}', key)
+            pattern = re.compile(pattern.lower())
+            rules[fieldname].append((pattern, value))
+            if fieldname == 'artist':
+                # Special case for the artist field: apply the same
+                # rewrite for "albumartist" as well.
+                rules['albumartist'].append((pattern, value))
+
+        # Replace each template field with the new rewriter function.
+        for fieldname, fieldrules in rules.items():
+            getter = rewriter(fieldname, fieldrules)
+            self.template_fields[fieldname] = getter
+            if fieldname in library.Album._fields:
+                self.album_template_fields[fieldname] = getter