SP metadata

Implementation of SP Metadata.
- Produces the bare mininum metadata required by the spec.
- Fixes #4.
Even though the specification says that EntityId is a URI I think it is still wise to keep the string datatype here, we don't have any specific benefits to convert to the URI datatype, and keeping it a string would allow the AuthServices to better operate in a non standards conforming environment.
My opinion is to keep it as a string, just wanted to raise the question if anyone else has another opinion.