RefExtract: selective ref section title detection

* We now set a priority on which title form is the most important.
  References will be given priority over anything else while Reference
  will ranked last with Bibliography in between.
  That means that if "Bibliography" is found in the text, we assume it is
  the reference section and we don't look for a section titles "Reference".
WebSearch: limit on 'refersto' and simular keywords

* Limit DOS attacks with 'refersto' and similar keywords (e.g.
  'refersto:year:0-->9999').
* Adds configuration variable CFG_WEBSEARCH_MAX_RECORDS_REFERSTO and
  CFG_WEBSEARCH_MAX_RECORDS_CITEDBY.
* Limits result used by 'refersto', 'referstoexcludingselfcites' to
  CFG_WEBSEARCH_MAX_RECORDS_REFERSTO.
* Limits result used by 'citedby', 'citedbyexcludingselfcites' to
  CFG_WEBSEARCH_MAX_RECORDS_CITEDBY.
* Adds classes 'InvenioWebSearchReferstoLimitError' and
  'InvenioWebSearchCitedbyLimitError'.
* Adds warning messages when the limit CFG_WEBSEARCH_MAX_RECORDS_* is reached.
* In BibRank adds 'input_limits' parameter to 'get_refersto_by_list',
  'get_refersto_hitset', 'get_cited_by_list', 'get_citedby_hitset' (default
  to None).
* Adds regression test for the query 'refersto:year:2000'.

Signed-off-by: Federico Poli <federico.poli@cern.ch>

Conflicts:
	modules/websearch/lib/search_engine.py
	modules/websearch/lib/websearch_regression_tests.py