Create core oauthlib wrapper to handle OAuth centrally

Problem is that, right now we have four OAuth libs:

• lib/oauthlib.php, already used by some repository plugins.
• mod/basiclti, the oauth impl. used by the "old" module.
• mod/imslti, used by the "new" module (copy of the previous).
• PECL's Oauth extension, with its own API and potentially also causing some conflicts.

In core:

• enrol/lti/ims-blti/OAuth.php
• mod/lti/OAuth.php
• lib/oauthlib.php

We need one unique, and well defined Oauth core API (surely a wrapper over PECL's and ims-dev ones) to be used everywhere. Fully tested.

Until then we are using some namespaces (see MDL-20534) for the IMS-LTI module as workaround. Once implemented, all uses above will be revisited.

Ciao