added default namespace to thesaurus
changed how search by keyword works so that it checks ids as well as keyword labels
added default namespace for each thesaurus to make editing thesauri easier

Previously when one added an element to a thesaurus they would add a keyword and then they needed to change the id to add a namespace.
If they didn't change namespace then when the thesaurus was restarted the namespace would be a file uri to the thesaurus file.  This
is a problem if the id was used anywhere for identifying the keyword.

Fixes: http://trac.osgeo.org/geonetwork/ticket/1190