Realtime updater design and configuration

This commit implements most of the things discussed in https://groups.google.com/forum/#!topic/opentripplanner-dev/-smm6eHj7dE.

Notable difference: because interfaces can't contain static methods in Java, the fromPreference "factory-method" is not implemented. Instead, the PreferencesConfigurable interface is still used.

The following things are implemented:

- A lot of updater classes are shuffled around to make things more organized.
- All updaters apply their changes to the graph via GraphUpdaterManager.execute using one thread.
- The GTFS-RT support is refactored to be more modular.
- A websocket GTFS-RT updater is added.
- Updaters are not instantiated via Reflection anymore.

Because most updaters didn't have unit-tests, I couldn't verify whether they all still work as expected.