Namespace plugin implementation

* Creates pelican.plugins
* Moves plugin related code under pelican.plugins
  * pelican.plugins.signals is now the location for signals, pelican.signals is kept
    for backwards compatibility
  * pelican.plugins._utils contains necessary bits for plugin discovery and loading.
    Logic from Pelican class is moved here. Pelican class now just asks for plugins
    and registers them
* Contains tests for old and new plugin loading

pelican/plugins/signals.py

@@ -0,0 +1,52 @@
+# -*- coding: utf-8 -*-
+from __future__ import print_function, unicode_literals
+
+from blinker import signal
+
+# Run-level signals:
+
+initialized = signal('pelican_initialized')
+get_generators = signal('get_generators')
+all_generators_finalized = signal('all_generators_finalized')
+get_writer = signal('get_writer')
+finalized = signal('pelican_finalized')
+
+# Reader-level signals
+
+readers_init = signal('readers_init')
+
+# Generator-level signals
+
+generator_init = signal('generator_init')
+
+article_generator_init = signal('article_generator_init')
+article_generator_pretaxonomy = signal('article_generator_pretaxonomy')
+article_generator_finalized = signal('article_generator_finalized')
+article_generator_write_article = signal('article_generator_write_article')
+article_writer_finalized = signal('article_writer_finalized')
+
+page_generator_init = signal('page_generator_init')
+page_generator_finalized = signal('page_generator_finalized')
+page_generator_write_page = signal('page_generator_write_page')
+page_writer_finalized = signal('page_writer_finalized')
+
+static_generator_init = signal('static_generator_init')
+static_generator_finalized = signal('static_generator_finalized')
+
+# Page-level signals
+
+article_generator_preread = signal('article_generator_preread')
+article_generator_context = signal('article_generator_context')
+
+page_generator_preread = signal('page_generator_preread')
+page_generator_context = signal('page_generator_context')
+
+static_generator_preread = signal('static_generator_preread')
+static_generator_context = signal('static_generator_context')
+
+content_object_init = signal('content_object_init')
+
+# Writers signals
+content_written = signal('content_written')
+feed_generated = signal('feed_generated')
+feed_written = signal('feed_written')