BibIndex: refactoring and author parent tokenizer

* Refactoring of BibIndex:
  implements inveniosoftware#1710

* Adds a generic parent tokenizer, useful for the dataset records:
  implements inveniosoftware#1885

* Adds 4 demo records for testing purpose

* Adds (and fixes) tests

More details:

* Refactoring of `task_run_core` in `bibindex_engine.py`: easier to
  follow, moves the decision of which records need to be reindexed to
  the tokenizers

* Fixes bug: the command `bibindex -w [index name] -m [date range]`
  previously was updating the `last_updated` field of the index

* Moves many of the index specific special cases from
  `bibindex_engine.py` to the tokenizers
  (e.g. `if index_name == "author": # add special tokens`)

* Moves some utility functions from `bibindex_engine.py` to
  `bibindex_engine_utils.py`

* Adds tokenizers `BibIndexAbstractParentTokenizer`,
  `BibIndexCanonicalAuthorTokenizer`,
  `BibIndexExactParentAuthorTokenizer`,
  `BibIndexParentAuthorTokenizer`,
  `BibIndexParentCanonicalAuthorTokenizer`

* Adds indexes `simpleauthor`, `canonicalauthor`, `parentauthor`,
  `parentcanonicalauthor`, `exactsimpleauthor`, `exactparentauthor`

* Adds fields `simpleauthor`, `exactsimpleauthor`

* Adds upgrade script for the database changes
  (uses function upgrade_atlantis)

* Transforms `author` and `exactauthor` indexes from regular to
  virtual index

* Adds file `bibindex_engine_tokenizer_regression_tests.py`

* Adds tests for tokenizers, for bibindex CLI, for bibindex utils

* Closes inveniosoftware#1662
  (BibIndex: --reindex should not zero last_updated)

* Edits the commands run by `--load-demo-records` in order to
  reindex the bibauthorid-dependant indexes after the `bibauthorid`
  task (for example `canonicalauthor` and `parentcanonicalauthor`)
1: D100 Docstring missing
21:1: F401 'deserialize_via_marshal' imported but unused
21:1: F401 'run_sql' imported but unused
23:1: F401 'nottest' imported but unused
38: D400 First line should end with '.', not 'e'
61: D101 Docstring missing
64:5: E303 too many blank lines (2)
64: D400 First line should end with '.', not '0'
73: D400 First line should end with '.', not '9'
79: D400 First line should end with '.', not '1'
85: D400 First line should end with '.', not 'd'
92: D400 First line should end with '.', not 'x'
98: D400 First line should end with '.', not 'x'
104: D400 First line should end with '.', not 'x'
111: D101 Docstring missing
114:5: E303 too many blank lines (2)
114: D400 First line should end with '.', not 'r'
123: D400 First line should end with '.', not 'c'
129: D400 First line should end with '.', not 'c'
135: D400 First line should end with '.', not 'x'