Character-escape unicode characters automatically

[maxcutler]

xmlrpclib won't auto-escape characters that are invalid in XML, which can cause operations to fail when strings contain non-ASCII characters.

This library knows enough about the fields to auto-escape/encode those which can contain arbitrary strings (e.g., WordPressPost.title). Can be as simple as text.encode('ascii', 'xmlcharrefreplace') when building struct.