DocExtract: several improvements

* Accepts Publications as reference section.

* Refactors book handling and search.
  (closes #959)

* Daemon extracts all added pdfs.

* Correctly handles PDF/A files.

* Adds LIGO and LHCP report numbers to the kb.

* Adds the filename of the extracted files in the 999C6f field.

* Converts arxiv urls to report numbers

* Adds support for hdl identifiers.

* Sets a priority on which title form is the most important.
  References will be given priority over anything else while Reference
  will rank 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".

* Improves error message when pdftotext is missing.

Signed-off-by: Alessio Deiana <alessio.deiana@cern.ch>
Tested-by: Samuele Kaplun <samuele.kaplun@cern.ch>
1: D400 First line should end with '.', not 't'
4: I102 copyright year is outdated, expected 2014 but got 2011
75:36: E126 continuation line over-indented for hanging indent
79:36: E127 continuation line over-indented for visual indent
80:36: E127 continuation line over-indented for visual indent
96: D103 Docstring missing
139: D103 Docstring missing
152:8: E127 continuation line over-indented for visual indent
154:8: E127 continuation line over-indented for visual indent
177: D400 First line should end with '.', not 'n'
202: D401 First line should be imperative: 'Thi', not 'This'
202: D400 First line should end with '.', not 'e'
202: D205 Blank line missing between one-line summary and description
202: D208 Docstring is over-indented
227: D401 First line should be imperative: 'Thi', not 'This'
227: D400 First line should end with '.', not 'e'
227: D205 Blank line missing between one-line summary and description
227: D208 Docstring is over-indented
252: D401 First line should be imperative: 'Thi', not 'This'
252: D400 First line should end with '.', not 'e'
252: D205 Blank line missing between one-line summary and description
252: D208 Docstring is over-indented
277: D401 First line should be imperative: 'Thi', not 'This'
277: D400 First line should end with '.', not 'e'
277: D205 Blank line missing between one-line summary and description
277: D208 Docstring is over-indented
340:37: E203 whitespace before ':'
341:37: E203 whitespace before ':'
342:37: E203 whitespace before ':'
343:37: E203 whitespace before ':'
344:37: E203 whitespace before ':'
353: D400 First line should end with '.', not 'n'
353: D205 Blank line missing between one-line summary and description
353: D208 Docstring is over-indented
374:12: E127 continuation line over-indented for visual indent
385:60: E227 missing whitespace around bitwise or shift operator
434:41: E126 continuation line over-indented for hanging indent
447:38: E126 continuation line over-indented for hanging indent
466: D103 Docstring missing
468:35: E203 whitespace before ':'
469:35: E203 whitespace before ':'
470:35: E203 whitespace before ':'
471:35: E203 whitespace before ':'
472:35: E203 whitespace before ':'
501:13: E128 continuation line under-indented for visual indent