Implement initiating logout from s2repoze plugin.

Adds a new repoze config option path_logout to the challenge decider
plugin. When a request is received on this url a global logout request
is initiated. Responses to this request is received on the
single_logout_service endpoint of the sp as configured in saml config
of the sp. Tested with only on IdP and only using HTTP_REDIRECT bindings

TODO: handle receiving logout requests on the single_logout_service endpoint
Allow graceful handling of auth_tkt cookies outliving saml clients cache

If for some reason the session cookie outlives the saml clients cache,
for instance if the webservice is restarted there could be an
inconsistent state where the user is authenticated but saml attributes
are missing and saml logout requests will fail. By using only saml2sp
as authenticator plugin and repoze.who 2.0 this little check will work
around that and require a new login in this case
Added support for receiving SAMLRequests on the single_logout endpoint
PEP-8
Move to a more noticeable position.
Merge pull request #43 from fredrikt/master

restore functionality
Merge branch 'master' of github.com:rohe/pysaml2
Made all exceptions inherit from a basic pySAML2 exception.
Should be mail and not email
InCommon entity categories added
Need text to be utf-8, also allow for signature check on local file.
Need node_name definition
Damn, forgot cert parameter.
Allow for specifying directories in which the xmlsec binary may be found.
Use xmlsec path information.
Use xmlsec path information.
Use same parameters.
If no name_qualifier is chosen use the IdPs entity id.
More robus.
Editorial