[additional feature] Add attribute policy filters and modifiers

[peppelinux]

the IdP should have a default attr policy and customized policy for each SP.
example in settings.py, where '' meand default, an entityID should apply only on the corresponding SP.

SAML_ATTRIBUTE_POLICIES = {
                   '':  [{'package': 'uniauth.policies',
                         'policy_name': 'regexp_match',
                         'attribute_name': 'schacHomeOrganization',
                         'kwargs': {'regexp':'\.*unical.it',}},
                        ]
}

At the same way some attribute rewriter

SAML_ATTRIBUTE_REWRITERS = {
                   'that.ugly.sp':  [{'package': 'uniauth.rewriters',
                         'rewriter_name': 'replace',
                         'attribute_name': 'schacHomeOrganization',
                         'kwargs': {'from_str': 'unical', 'to_str': 'lacinu',}},

                          {'package': 'uniauth.rewriters',
                           'rewriter_name': 'regexp_replace',
                           'attribute_name': 'schacHomeOrganization',
                           'kwargs': {'regexp': 'unical', 'sub': 'gnocc',}},

                          {'package': 'uniauth.rewriters',
                           'rewriter_name': 'add_static_attribute',
                           'attribute_name': 'schacHomeOrganization',
                          'kwargs': {'value': 'ingoalla',}}
                        ]
}

rewriters result example:

schacHomeOrganization: unical.it
schacHomeOrganizationType: university
schacHomeOrganizationType: educationInstitution

if accounts doesn't have some common attribute values.

policies result example:
Filter out an Affiliation without a value "member"