Aggregator

The aggregator is called `agherant`. It is implemented as a [Blueprint](http://flask.pocoo.org/docs/0.10/blueprints/) for maximum flexibility (it can be reused even without the whole libreant).

The aggregator itself can be aggregated, using `/agherant/search?format=opensearch`.

This is not really, really complete:
* the `description_list` is still static so it only aggregates itself
* the code is not optimized at allo:
** no parallel running of query
** no timeouts
** no caching of descriptions

I believe that this can already be merged, and optimizations be postponed.