Replace os.sep.join with the more robust os.path.join

From the Python docs for os.sep [1]:

  Note that knowing this is not sufficient to be able to parse or
  concatenate pathnames - use os.path.split() and os.path.join()...

Where I touched a line, I also changed double quoted string literals
to single quotes, since they are used more often in the source:

  wking@mjolnir ~/src/pelican $ git grep "'" pelican/*.py | wc -l
  683
  wking@mjolnir ~/src/pelican $ git grep '"' pelican/*.py | wc -l
  181

[1]: http://docs.python.org/3/library/os.html#os.sep

pelican/generators.py

@@ -347,7 +347,7 @@ class ArticlesGenerator(Generator):
     def generate_drafts(self, write):
         """Generate drafts pages."""
         for article in self.drafts:
-            write('drafts/%s.html' % article.slug,
+            write(os.path.join('drafts', '%s.html' % article.slug),
                 self.get_template(article.template), self.context,
                 article=article, category=article.category,
                 all_articles=self.articles)