ConfigurationView (hence Settings) is a MutableMapping

The <code>celery.datastructures.ConfigurationView</code> acts like a dictionary, so it should register itself as such with the <code>MutableMapping</code> abstract base class. This is useful for code which detects dict-like things.

(Many of the <code>collections</code> abstract base classes, such as <code>Iterable</code>, implement <code>subclasshook</code> to define themselves via duck-typing, checking for implemented methods. Unfortunately <code>Mapping</code> and <code>MutableMapping</code> do not do this, so it is necessary to explicitly register.)

Includes an implementation of <code>len</code>, to complete the interface, and unit tests.

Also includes what I believe is a correction of misleading documentation -- it appears that in <code>ConfigurationView(changes, defaults)</code>, <code>defaults</code> is a list of dicts, not a dict.