[dataimport] fix db-namespace default value handling
In a fresh new instance, the "db-namespace" option is set as empty
by default in the "sources" file.
This seems correct but empty string is not a valid pg_schema and
actually means that we should use "public"
The facets system
-----------------
Facets allow to restrict searches according to some user friendly criterias.
CubicWeb has a builtin `facet`_ system to define restrictions `filters`_ really
as easily as possible.
Here is an exemple of the facets rendering picked from our
http://www.cubicweb.org web site:
.. image:: ../../images/facet_overview.png
Facets will appear on each page presenting more than one entity that may be
filtered according to some known criteria.
Base classes for facets
~~~~~~~~~~~~~~~~~~~~~~~
.. automodule:: cubicweb.web.facet
.. _facet: http://en.wikipedia.org/wiki/Faceted_browser
.. _filters: http://www.cubicweb.org/blogentry/154152