Add support for new MobiusCore. Fixes #1231

You must update to the latest MobiusCore_1.2.2_1.6.4 or Opis will not work.

* Add NMS remapping support for PluginClassLoader's findClass.  Fixes #1253

In some cases, plugins may make direct calls to the PluginClassLoader
findClass method. Instead of throwing an exception, it will now locate the
mapping and pass it off to Forge's LaunchClassLoader.

* Change CraftServer.serverName to "Cauldron-MCPC-Plus" to maintain
compatibility with plugins looking for old name.