Rework plugin infrastructure to nix side-effects

This reworks the plugin infrastructure so as to remove module-loading
side-effects which were making things a pain in the ass to test.

With the new system, there's no auto-registering meta class. Instead
plugins do whatever they want and then specify a hooks dict that maps
hook names to callables for the things they're tying into. The most
common one (and the only one we've implemented so far) is "setup".

This also simplifies the sampleplugin a little by moving the code
to __init__.py.

docs/source/pluginwriter/quickstart.rst

@@ -50,7 +50,8 @@ The inner ``sampleplugin`` directory is the Python package that holds
 your plugin's code.
 
 The ``__init__.py`` denotes that this is a Python package. It also
-holds the plugin code.
+holds the plugin code and the ``hooks`` dict that specifies which
+hooks the sampleplugin uses.
 
 
 Step 2: README
@@ -107,43 +108,39 @@ The code for ``__init__.py`` looks like this:
 
 .. code-block:: python
    :linenos:
-   :emphasize-lines: 8,19
+   :emphasize-lines: 12,23
 
     import logging
     from mediagoblin.tools.pluginapi import Plugin, get_config
 
 
+    # This creates a logger that you can use to log information to
+    # the console or a log file.
     _log = logging.getLogger(__name__)
 
 
-    class SamplePlugin(Plugin):
-        """
-        This is a sample plugin class. It automatically registers itself
-        with MediaGoblin when this module is imported.
-
-        The setup_plugin method prints configuration for this plugin if
-        there is any.
-        """
-        def __init__(self):
-            pass
-
-        def setup_plugin(self):
-            _log.info("I've been started!")
-            config = get_config('sampleplugin')
-            if config:
-                _log.info('%r' % config)
-            else:
-                _log.info('There is no configuration set.')
+    # This is the function that gets called when the setup
+    # hook fires.
+    def setup_plugin():
+        _log.info("I've been started!")
+        config = get_config('sampleplugin')
+        if config:
+            _log.info('%r' % config)
+        else:
+            _log.info('There is no configuration set.')
 
 
-Line 8 defines a class called ``SamplePlugin`` that subclasses
-``Plugin`` from ``mediagoblin.tools.pluginapi``. When the class is
-defined, it gets registered with MediaGoblin and MediaGoblin will then
-call ``setup_plugin`` on it.
+    # This is a dict that specifies which hooks this plugin uses.
+    # This one only uses one hook: setup.
+    hooks = {
+        'setup': setup_plugin
+        }
 
-Line 19 defines ``setup_plugin``. This gets called when MediaGoblin
-starts up after it's registered all the plugins. This is where you can
-do any initialization for your plugin.
+
+Line 12 defines the ``setup_plugin`` function.
+
+Line 23 defines ``hooks``. When MediaGoblin loads this file, it sees
+``hooks`` and registers all the callables with their respective hooks.
 
 
 Step 6: Installation and configuration