elsevier_package: fixes on _add_references function

* Adds a new case for the label harvesting to remove
  the ending parenthesis.
elsevier_package: fix bad os call
Fixes imports and requirements

* setup: now also installs the requirements
  from requirements.txt
* elsevier_package: fixes imports
* requirements: adds a file to specify requirements
Merge pull request #3 from GiorgosPa/consyn-harvest

elsever_package.py: Fixes on _add_references function
Merge pull request #4 from GiorgosPa/import-fixes

Fixes imports and requirements
elsevier_package: Various fixes for CONSYN harvesting

* ignores license
* fixes date harvesting
* fixes issue harvesting
* fixes journal name
* fixes reference harvesting for book-series
  and conferences
Merge pull request #5 from GiorgosPa/author-harvesting

elsevier_package: Various fixes for CONSYN harvesting