Added Bug.from_xml() + some .from_xml() fixups.

Added Bug.from_xml() + some .from_xml() fixups.

Moved comment.InvalidXML to utility.InvalidXML, so that bug and
comment can share it.  Added docstring explaining the __init__
arguments.

Added indent and shortname options to Bug.xml() to match
Comment.xml().

Added .extra_strings export to Comment.xml().

Converted Bug.xml() from string addition to list joining, which avoids
a bunch of memory allocation/deallocation.

Assorted " -> ' replacements.

Elaborated doctests to check UTF-8, extra_strings, ...

Added new comparison cmp_extra_strings for both bug. and
comment.DEFAULT_CMP_FULL_CMP_LIST.