Added allow_unsolicited parameter to config class.
As Rainer Hörbe pointed out: "A principal is a user, not an entity in SAML. The UnknownPrincipal exception in mdstore.py should be renamed as suggested in the title to avoid confusion.
Unfortunately the OASIS SAML glossary is using the X-811 definition of principal, but contradicts itself otherwise by redefining the term implicitly in other definitions such as Account, Affiliation, IDP, Persistent Pseudonym, etc"
Merge branch 'master' of github.com:rohe/pysaml2