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: D100 Docstring missing
4: I102 copyright year is outdated, expected 2014 but got 2013
23:39: E126 continuation line over-indented for hanging indent
38: D103 Docstring missing
42: D400 First line should end with '.', not 'a'
42: D205 Blank line missing between one-line summary and description
42: D208 Docstring is over-indented
86:12: E126 continuation line over-indented for hanging indent
86:32: E203 whitespace before ':'
87:32: E203 whitespace before ':'
88:32: E203 whitespace before ':'
89:32: E203 whitespace before ':'
90:32: E203 whitespace before ':'
91:32: E203 whitespace before ':'
92:32: E203 whitespace before ':'
104: D400 First line should end with '.', not 't'
118:15: E127 continuation line over-indented for visual indent
119:15: E127 continuation line over-indented for visual indent
120:46: E128 continuation line under-indented for visual indent
123: D103 Docstring missing
127: D103 Docstring missing
136: D103 Docstring missing
144: D290 White spaces around docstring should be trimmed.
144: D400 First line should end with '.', not 'h'
144: D205 Blank line missing between one-line summary and description
144: D208 Docstring is over-indented
235: D103 Docstring missing