Add MultimapStream utility class to convert MapStreams to/from Multimaps

Google commons is added as an optional dependency; no runtime dependency on
google commons should be present unless the MultimapStream class is referenced.
Use SimpleImmutableEntry because we don't want or need mutable entries
Add utility methods for creating a MapStream from a Stream. Add some javadoc.
Javadoc
Use convenience methods on Entry for comparing by key/value