extended mapping_class for config to function & context

the method ```ConfigurationManager.get_config``` accepts an object hook so that the users can specify what type of mapping container in which they want their configuration.  That method isn't the only way to get configuration out of configman.  

This PR extends the mapping object_hook to the two other ways to get configuration:  ```ConfigurationManager.context``` and the bare function ```configuration```
because the user could have specified that they want their config in the form of ```dict``` instead of a Mapping that overloads the dot operator.  I must take that into account.