Add extensibility about nether

[stuartleeks]

PR #68 starts this work by enabling the dependencies to be loaded from config

Still to do:

• remove dependency in project.json (requires updating the build to copy the dependencies)
• handle "implementation" configuration option as well as "factory"
• review the approach - e.g. should we allow shared config file across modules (probably depends on how we deploy), should we allow modules to register dependencies as an alternative approach (e.g. MongoStore implementation could add a MongoClient dependency that is consumed by the store)

[stuartleeks]

I think we've got a config/DI solution that is working well enough for us at the moment.

Closing this, but open specific issues for any problems/enhancements