Fix an issue in neighbors plugin which was introduced after subcategory support

It is not necessary that articles will have subcategories. You may
end up getting following critical error.

>
CRITICAL: ("'ArticlesGenerator' object has no attribute 'subcategories'",)
CRITICAL: 'ArticlesGenerator' object has no attribute 'subcategories'
Traceback (most recent call last):
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/bin/pelican", line 8, in <module>
    load_entry_point('pelican==3.3', 'console_scripts', 'pelican')()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/__init__.py", line 346, in main
    pelican.run()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/__init__.py", line 162, in run
    p.generate_context()
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/pelican-3.3-py2.7.egg/pelican/generators.py", line 480, in generate_context
    signals.article_generator_finalized.send(self)
  File "/Users/talha/Repos/VirtualEnvs/pelican-dev/lib/python2.7/site-packages/blinker/base.py", line 267, in send
    for receiver in self.receivers_for(sender)]
  File "/Users/talha/Repos/pelican-plugins/neighbors/neighbors.py", line 49, in neighbors
    for subcategory, articles in generator.subcategories:
AttributeError: 'ArticlesGenerator' object has no attribute 'subcategories'
Merge pull request #149 from talha131/fix-neighbors

Neighbors plugin shouldn't require subcategories
Fixed missing quote mark in an HTML example
Allow for an optional maximum number of entries to display.
Merge pull request #152 from mlang/master

Add optional max number of entries to github_activity plugin
Merge pull request #150 from runarberg/master

Fix missing quote mark in subcategories plugin docs
Simple Footnotes plugin. Add footnotes to your text by enclosing them in [ref] and [/ref].
Merge pull request #156 from stuartlangridge/simple-footnotes

Add Simple Footnotes plugin
Initial release of the twitter bootstrap rst directives plugin
Merge pull request #157 from samyarous/master

Pelican plugin providing RST directives for twitter bootstrap partial intergration
new latex math plugin, now handles typogrify correctly and does not need template alteration to be used
Update Readme.md

Fixed spelling error
Merge pull request #161 from barrysteyn/master

latex plugin revamp
Logic for MathMl; Fixed Bug; Updated Readme
Fixed slight error with mathml regex in summary
Merge pull request #162 from barrysteyn/master

Logic for MathMl; Fixed Bug; Updated Readme
moved latex to render_math; latex now symbolic link
Merge pull request #166 from barrysteyn/master

latex plugin name changed to render_math
ability to specify ssl protocol
Merge pull request #169 from barrysteyn/master

Add ability to specify SSL protocol