Rename PluginLoader to LegacyPluginLoader
Replace custom plugin loader with Java ServiceLoader

The previously used mechanism for loading plugins into Graylog2 was reimplementing
the functionality provided by the standard Java [ServiceLoader][1] facility.

Plugin authors can now bundle multiple plugins in one single JAR artifact. Instead
of creating a graylog2-plugin.properties file, plugin authors have to create a
file named META-INF/services/org.graylog2.plugin.Plugin which contains a list of
classes implementing the Plugin interface (one FQCN per line).

[1]: http://docs.oracle.com/javase/7/docs/api/java/util/ServiceLoader.html
Merge branch 'plugin-serviceloader'