WebAccess: ORCID support
WebAccess: ORCID support

* Introduces support for OAuth2-based ORCID authentication by exploiting
  the external authentication support to OpenID/OAuth1/OAuth2 introduced
  with the previous commit.

* Amended previous commit by cleaning up INSTALL guide and root-level
  Makefile.am, introduced configuration of providers in invenio(-local).conf.

* In the previous commits, the new provided plugins were implemented
  assuming to be instantiated per-every request, while each plugin
  is instantiated only upon server loading and can't hence share
  request-oriented information. For this reason a new attribute called
  "g" (mimicking upcoming Flask global "g" proxy) to the req object.
  (closes #1124)

Tested-by: Tibor Simko <c7116e45300b163846dc01f3ed02116e3f8ef7a5@cern.ch>

inveniosoftware/invenio-base,tiborsimko/invenio-base,inveniosoftware/invenio-base,tiborsimko/invenio-base