move beaker to an optional/extras requirement under 'examples'

[biyeun]

There is a CVE out for beaker with no option to upgrade to a fixed version as the project appears largely unmaintained.

It appears that beaker is only used in the examples and is therefore not a necessary requirement to use oic. This change moves beaker to extras_requires in setup.py under examples, so that this package is not unnecessarily installed as a required sub-dependency on projects listing oic as a dependency.

[schlenk]

The CVE is kind of bogus, as one can use a json_serializer to enforce json for storage instead of pickle, so it is not unmitigated.
But yes, beaker looks pretty unmaintained (bbangert/beaker#214 is the proper statement) and has other issues that could be seen as security hazards (e.g. contradictory documentation how to actually invalidate/delete a session after logout.).

But i agree that beaker is only needed for examples, so should not be needed for the base package.

[schlenk]

LGTM.

Thanks for the PR.