Update Git submodules.

* Update Boost and Urdl libraries.
* Remove broken submodule at '3rd.party/termbox'.
Insignificant modifications made in 'core' & 'python' libraries.

* Auto-register aggregators (works only if building shared libraries).
* Tiny modifications to 'audiopia::resource' & 'audiopia::collection'.
* 2nd argument of the Python 'audiopia.collection' class's constructor is optional.
Modify Python bindings.

Fix issue when a Python aggregator does not implement more().
The aggregate() method will be trigger only one time and won't loop over
and over again.
Add empty Jamfile for 3rd.party project 'termbox'.
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.
Fix MongoDB problem with last Boost version.
Add example 'query'.
Modify aggregation framework.

Aggregator initializer are registered in aggregator_factory.cpp.
Modify Python bindings.