Add "ready" callback function support

[funkybob]

Currently due to a implementation detail the class is only instantiated once when invoking Settings.use().

If this were formalised, and an optional ready() method specified, we could allow a post-settings-selection callback to be guaranteed invoked.

One issue with this is allowing access to the full set of global settings and defaults, if necessary.

Given currently django iterates the module using dir() we could invoke ready() after that method has been exhausted?