Fixed tipue_search failing with pages due to no category.

Would fail with following error on pages due to pages having no
category:

    CRITICAL: 'Page' object has no attribute 'category'
    Traceback (most recent call last):
      File "/usr/local/bin/pelican", line 9, in <module>
        load_entry_point('pelican==3.2', 'console_scripts', 'pelican')()
      File "/usr/local/lib/python2.7/dist-packages/pelican-3.2-py2.7.egg/pelican/__init__.py", line 376, in main
        pelican.run()
      File "/usr/local/lib/python2.7/dist-packages/pelican-3.2-py2.7.egg/pelican/__init__.py", line 181, in run
        p.generate_output(writer)
      File "/home/rmore/src/pelican-plugins/tipue_search/tipue_search.py", line 66, in generate_output
        self.create_json_node(page)
      File "/home/rmore/src/pelican-plugins/tipue_search/tipue_search.py", line 43, in create_json_node
        if getattr(page, 'category') == 'None':
    AttributeError: 'Page' object has no attribute 'category'