Added be-mbox-to-xml.

Added be-mbox-to-xml.

Reworked to allow "be comment" to handle unicode strings (see bug
e4ed63f6-9000-4d0b-98c3-487269140141).  The solution was to escape all
the unicode to produce and ASCII string before calling
ElementTree.XML, and then converting back to unicode afterwards.

Added a unicode-containing comment to the end of bug
f7ccd916-b5c7-4890-a2e3-8c8ace17ae3a so that there's a handy unicode
comment for testing.

XML headers (e.g. '<?xml version="1.0" encoding="UTF-8" ?>') are
now added to all xml output from be.

Switched non-text/* encoding library to base64 instead of
email.encoders, which makes that code in libbe/comment.py simpler.

Changed libbe/mapfile.py error encoding from string_escape to
unicode_escape so it can handle unicode.

Everything's still untested, and be-xml-to-mbox doesn't handle unicode
yet, but I felt this commit was getting a bit unwieldy ;).