PluginIndex: allow null PluginFinder

The PluginFinder of a PluginIndex can now be null. If null, no plugins
will be discovered during the discover() method call.
Add ScijavaService interface

Added a top-level ScijavaService interface, that all other
scijava-common services extend.

This allows Contexts to be created with just the ScijavaService, and
have all service subclasses discovered automatically.
ContextCreationTest: add new tests

Added test coverage for the new API added to Context and ServiceHelper.