Modify 'db' and 'listdir' examples.

* Add a try-catch block in 'listdir' example.
* Modify 'db' example to take any number of arguments to build the query.
Modify aggregation framework.

* Remove aggregator initializers for proxy collections (buffer, group, filter, sort, bucket).
* Classes aggregator_factory, aggregator_initializer and query_collection modified to take different arguments.
* Change sort algorithm in sort_collection to support inner-field sorting.
* Move the sort algorithm in sort_collection to sort on aggregation.
Adjust Python bindings to last modifications

* The runtime_inititializer class takes *args and **args.
* The Python 'audiopia.collection' class takes *args and **args.