Change sort for NamedExtensionManager

Use a less expensive algorithm for ordering the
extensions.

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Add ExtensionManager.__getitem__

Allow extensions to be accessed directly by name.

Fixes issue #15

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Clean up autodoc for manager classes

Use default settings instead of repeating the options
for each class.

Add docstrings for special methods that do not have them
so they appear in the output.

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>