Fix TypeError when parsing tags like <img src='foo' alt>

In this case, doing attrs.get('alt', '') would return None, because attrs
actually looks like {'src': 'foo', 'alt': None}. This caused an unhandled
TypeError in escape_md when trying to do regular-expression matching on the
None value.
Fix TypeError when attrs['href'] is None in handle_tag.
Merge pull request #25 from inboxapp/master

Fix TypeError when parsing tags like <img src='foo' alt>