Use whoosh as search engine for looking for users and groups

Signed-off-by: Álvaro Arranz <aarranz@fi.upm.es>
Refactor catalogue search engine

Signed-off-by: Álvaro Arranz <aarranz@fi.upm.es>
Wirecloud search engines work with buffered writers

Signed-off-by: Álvaro Arranz <aarranz@fi.upm.es>