Keeping not JSON serializable objects out of html context (#47)

ABlog objects are not JSON serializable. They are used for
creating archive pages, and work fine with HTML builders.
When json builder is used, it pickles HTML context along
with HTML, and this causes serialization issues. So ABlog
objects (functions, Catalog, Blog instance, etc.) are avoided
when builder is not one of html/dirhtml.

ablog/__init__.py

@@ -22,6 +22,7 @@ def anchor(post):
 
 def html_page_context(app, pagename, templatename, context, doctree):
 
+    if app.builder.name in {'html', 'dirhtml'}:
         context['ablog'] = Blog(app)
         context['anchor'] = anchor
 

ablog/post.py

@@ -361,7 +361,7 @@ def process_postlist(app, doctree, docname):
     """Replace `PostList` nodes with lists of posts. Also, register all posts
     if they have not been registered yet."""
 
-    blog = Blog()
+    blog = Blog(app)
     if not blog:
         register_posts(app)
 
@@ -480,6 +480,9 @@ def generate_archive_pages(app):
     """Generate archive pages for all posts, categories, tags, authors, and
     drafts."""
 
+    if app.builder.name not in {'html', 'dirhtml'}:
+        return
+
     blog = Blog(app)
     for post in blog.posts:
         for redirect in post.redirect:
@@ -559,6 +562,9 @@ def generate_atom_feeds(app):
     """Generate archive pages for all posts, categories, tags, authors, and
     drafts."""
 
+    if app.builder.name not in {'html', 'dirhtml'}:
+        return
+
     blog = Blog(app)