Add Baseclass for ClientDatabase

[tpazderka]

There is no BaseClass for implementation of custom client databases. We should create one that defines the basic expected API.

The class should probably reside in oic.utils.cdb and the basic API can be based on oic.utils.client_management.CDB object which can then inherit from the new class.

The basic expected API should be along these lines:

class BaseClientDatabase(object)
    def __init__(self)
    def get(self, item, default=None)
    def __getitem__(self, item)
    def __setitem__(self, item, value)
    def __delitem__(self, key)
    def __contains__(self, item)
    def keys(self)
    def items(self)
   def __len__(self)

The __init__ methods in providers can then warn if the cdb argument doesn't inherit from the new BaseClientDatabase.