oaiharvest: reliability improvement and docs

* Prints not only error logs for every workflow, but also
  everything with a superior code error (40) like critical.

* Removes useless part of code.

* Adds basic sphinx documentation syntaxes to various functions.

* Reactivates mails and tickets for harvesting process.

Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>
Signed-off-by: Guillaume Lastecoueres <guillaume.lastecoueres@cern.ch>
docs: how to develop modules addition

* Adds some documentation on how to develop modules in Invenio
  v2.0.0. Still work in progress.

* Fixes typo in extensions.rst and introduction.rst.

Signed-off-by: Jan Aage Lavik <jan.age.lavik@cern.ch>