Route Providers, Modularity and configurable Discovery

Major project refactor, please read docs while upgrading from previous versions!

• Akka Management is now an aggregator module, you need to include Akka Cluster HTTP Management explicitly.
• Akka Discovery introduced, implementation backed by Akka IO's DNS
  • more implementations expected to come from the community
• Akka Management now is able to automatically expose routes from other extensions, via so called Route Providers, read docs for more details.
• Akka Bootstrap is now perfected a bit more an documented extensively, it uses any Discovery method that you want.