Add plugin loading facilities as well as a small demo application to idbc-core:

This adds the following classes:

- PluginManager for loading and instantiating implementations of
  ImageDatabaseReader and ImageDatabaseWriter from JAR or CLASS files in an
  arbitrary plugin directory.

- IDBCDemo: A small CLI demo application that takes a plugin directory and
  lists all readers and writers that it could load using the PluginManager.