Introduce new plugin system.

The new system is backward compatible and will load older style plugins.

The new plugin framework allows plugins to track their dependencies, and
will auto-enable plugins as needed.

Dependencies are tracked via a class-level set named `dependencies` in
each plugin.

Plugin names are no longer tightly coupled with the plugin class name,
Pso EP8 style class names may be used.

Disabling plugins is now allowed, but ensuring proper cleanup is left to
the plugin implementation.

The use of a `post_init()` method is no longer needed for new style
plugins, but plugins following the old style will still require a
`post_init()` method.

sleekxmpp/plugins/__init__.py

@@ -6,6 +6,9 @@
     See the file LICENSE for copying permission.
 """
 
+from sleekxmpp.plugins.base import PluginManager, PluginNotFound, \
+                                   BasePlugin, register_plugin
+
 __all__ = [
     # Non-standard
     'gmail_notify',  # Gmail searching and notifications