Added RiverClassLoader. A substitute for RMIClassLoader, 
replaceable via a Spi implementation of net.jini.loader.RiverClassLoaderSpi.
Default behaviour is using the RMIClassLoader.

Added LocalHostLookup. Determines the hosts network identity.
Pluggable via SPI.
Default behaviour is using InetAdress.getLocalHost().

Introduced @Beta annotation.