WebSearch: new Add-to-Search Interface

* New search box interface that contains both simple search
  and advanced search (javascript dependent) (addresses #622).

* Replaces the 3-line advanced search, with only 1-line add-to-search,
  with the possibility of adding patterns to the current search pattern.

* Ammends regression tests to take into account a third search
  interface.

* Moves the config variables CFG_WEBSEARCH_ENABLED_SEARCH_INTERFACES and
  CFG_WEBSEARCH_DEFAULT_SEARCH_INTERFACES from the inveniocfg.py to
  invenio.config (closes #271)

Signed-off-by: Ludmila Marian <ludmila.marian@gmail.com>