Have relevant plugins use all_generators_finalized signal

clean_summary, read_more_link, representative_image, summary
plugins now use all_generators_finalized signal so as to avoid
issue #314. They fallback to current behavior is said signal is
not yet available.

See also: getpelican/pelican#1616

Closes #314.

summary.py

@@ -8,6 +8,7 @@
 
 from __future__ import unicode_literals
 from pelican import signals
+from pelican.generators import ArticlesGenerator, StaticGenerator
 
 def initialized(pelican):
     from pelican.settings import DEFAULT_CONFIG
@@ -21,7 +22,7 @@ def initialized(pelican):
         pelican.settings.setdefault('SUMMARY_END_MARKER',
                                     '<!-- PELICAN_END_SUMMARY -->')
 
-def content_object_init(instance):
+def extract_summary(instance):
     # if summary is already specified, use it
     # if there is no content, there's nothing to do
     if hasattr(instance, '_summary'):
@@ -35,7 +36,6 @@ def content_object_init(instance):
     begin_marker = instance.settings['SUMMARY_BEGIN_MARKER']
     end_marker   = instance.settings['SUMMARY_END_MARKER']
 
-    # extract out our summary
     content = instance._content
     begin_summary = -1
     end_summary = -1
@@ -69,6 +69,22 @@ def content_object_init(instance):
     instance._summary = summary
     instance.has_summary = True
 
+
+def run_plugin(generators):
+    for generator in generators:
+        if isinstance(generator, ArticlesGenerator):
+            for article in generator.articles:
+                extract_summary(article)
+        elif isinstance(generator, PagesGenerator):
+            for page in generator.pages:
+                extract_summary(page)
+
+
 def register():
     signals.initialized.connect(initialized)
-    signals.content_object_init.connect(content_object_init)
+    try:
+        signals.all_generators_finalized.connect(run_plugin)
+    except AttributeError:
+        # NOTE: This results in #314 so shouldn't really be relied on
+        # https://github.com/getpelican/pelican-plugins/issues/314
+        signals.content_object_init.connect(extract_summary)