Support SASL Authentication Against Kerboros

Need to support authentication against kerboros stores and then communicate tickets with mongodb using the SASL protocol.

[Note added 2013-01-04]:

As part of this effort the API for authentication had to change. You used to provide a different credential for each database you worked with. We are moving to a simpler model where you provide a list of credentials to MongoClientSettings and every connection is authenticated against all the credentials provided. The recommended approach is to define a single user in MongoDB for each application and give that user access to just the databases that the application needs. This would simplify the client side to the point where only a single credential need be supplied.

Also as part of this effort we are standardizing on Credential (no "s") for singular and Credentials for plural. The MongoCredentials class is now called MongoCredential, which is a breaking change, but the MongoCredential class had to be totally redesigned anyway so it was already breaking.