CSHARP-603: initial commit for SASL authentication.
CSHARP-603: refactoring.  Full support for SecureString.
CSHARP-603: added support for setting credentials via url.
cleaning up warnings.
added support for Negotiate Credentials into the type system and MongoUrlBuilder.
reorganized.
added support for negotiation.
fixed some namespaces and introduced the concept of a mechanism factory.
renaming, refactoring.
added support for DIGEST-MD5.
documentation.
added in MongoIdentity and MongoIdentityEvidence classes.
changed credentials to support the new identity and evidence classes.
replaced MongoClientIdentity in Sasl code.
working through url builders and settings objects and tests.
changed Negotiate to Strongest, authType to authProtocol, and made admin the default database from connection strings.
removed MongoClientIdentity.
refactored authentication logic to be union of privileges.
cleaning up warnings.
removed all items related to credentials from MongoServer, MongoDatabase, and MongoDatabaseSettings.