Support non-ASCII characters in multi-line strings.  Fixes #49.

 * pyrseas/dbobject/__init__.py (DbObject.__init__): For Python 2 and
   multi-line strings, decode the strings from UTF-8.
 * pyrseas/dbobject/{cast, extension, foreign, language, schema}.py
   ({CastDict, ExtensionDict, ForeignDataWrapperDict, LanguageDict,
   Schema}.to_map: Replace yaml.dump with yamldump.
 * pyrseas/dbtoyaml.py: Replace yaml.dump with yamldump.
 * pyrseas/yamlutil.py (MultiLineStr): For Python 2, derive the class
   from unicode, rather than str.  (yamldump): New function to enforce
   common defaults for YAML dumps.