Fixed class loader of the script engine.

The script engine didn't have the plugin class loader. Instead, it had the thread context class loader. The problem with this is that this prevents scripts from instantiating classes from other plugins to e.g. implement undo when WorldEdit is available. The thread context class loader is now set to the plugin class loader while instantiating the script engine which solves this issue.
Merge pull request #154 from pvginkel/master

Fixed class loader of the script engine.