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>