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/tests/dummy_plugins/namespace_plugin/pelican/plugins/ns_plugin/__init__.py

@@ -0,0 +1,5 @@
+NAME = 'namespace plugin'
+
+
+def register():
+    pass

pelican/tests/dummy_plugins/normal_plugin/normal_plugin/__init__.py

@@ -0,0 +1,5 @@
+NAME = 'normal plugin'
+
+
+def register():
+    pass