BibIndex: move helper functions to utils file

* Moves some of the helper functions from bibindex_engine
  to bibindex_engine_utils file.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
BibIndex: WordTable API changes

* Removes 'fields_to_index' argument from WordTable's argument
  list.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
BibIndex: abstraction layer for terms retrieval

* Introduces new TermCollector class which
  handles terms retrieval from different
  part of the system.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
BibIndex: indexing non-MARC standards
* Introduces NonMarcTermCollector for acquiring data
  from non-MARC records.

* Adds new column to tag table and inserts
  corresponding RecJson fields as another
  non-MARC value for 'tag'.

* Enhances test suite acordingly.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>
BibIndex: recjson fields in admin interface

* Admin interface shows recjson fields
  in logical fields overview.

* Fields and tags are shown directly in index
  overview.

* Edition of tag enhanced with option to
  change value for recjson field.

Signed-off-by: Grzegorz Szpura <grzegorz.szpura@cern.ch>