Added comment import to Bug.from_xml().

Added comment import to Bug.from_xml().

This is a pretty critical feature, dunno how I missed it before.

I also added a little check to both Bug and Comment.from_xml() so that
xml_string can take an ElementTree Element as well as the usual raw
string/unicode.  This avoids repeated string <-> Element conversions.

Added Bug.add_comment() which handles the addition of a Comment
instance, matching .in_reply_to, checking .uuid uniqueness, etc.