WebSearch: limit on 'refersto' and similar keywords

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 = no limit).
* Adds regression test for the query 'refersto:year:2000'.

Signed-off-by: Federico Poli <federico.poli@cern.ch>
2: I102 copyright year is outdated, expected 2014 but got 2012
33:42: E231 missing whitespace after ','
33:53: E231 missing whitespace after ','
33:61: E231 missing whitespace after ','
49:1: E302 expected 2 blank lines, found 1
49: D203 Expected 1 blank line *before* class docstring, found 0
49: D204 Expected 1 blank line *after* class docstring, found 0
54:5: E301 expected 1 blank line, found 0
58:1: E302 expected 2 blank lines, found 1
58: D203 Expected 1 blank line *before* class docstring, found 0
58: D204 Expected 1 blank line *after* class docstring, found 0
64:1: E302 expected 2 blank lines, found 1
64: D203 Expected 1 blank line *before* class docstring, found 0
64: D204 Expected 1 blank line *after* class docstring, found 0
64: D200 One-line docstring should not occupy 3 lines
72:1: E302 expected 2 blank lines, found 1
72: D203 Expected 1 blank line *before* class docstring, found 0
72: D204 Expected 1 blank line *after* class docstring, found 0
72: D200 One-line docstring should not occupy 3 lines