Allow README.TXT files in metadata directory

[JornWildt]

Should anyone, by accident or on purpose, put a non-metadata file in the metadata file directory, the OIOSAML simply breaks down.

We like to add a README.TXT in the directory to explain it's purpose, and for this we modified SAML20FederationConfig ever so slightly with an added file mask of "*.config":

internal void Initialize()
  string[] files = Directory.GetFiles(MetadataLocation, "*.config");

In addition to this, it would be nice if OIOSAML handled non-metadat files by signalling a clear error message, indicating what is wrong.